Inflorescence

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed.

1. INFLORESCENCE

2. Flowers are the organs of sexual reproduction in higher plants. They are, in fact, modified shoots, specialized for sexual reproduction. They may occur either singly (solitary flowers) or in bunches on a common axis. Solitary flowers may be axillary or terminal. Axillary flower is a modified axillary bud, and terminal flower is a modified terminal bud. An axis, bearing a bunch of flowers is known as inflorescence. It may be axillary or terminal in position.

3. The axial stalk or the main axis of the inflorescence is called rachis or peduncle . The individual flowers of an inflorescence are attached to the peduncle by stalks of their own, known as the pedicels . In some cases, flowers are stalkless and are sessile flowers, e.g., Achyranthes . In some plants like Lotus, and Onion, a long peduncle arises from the underground stem and bears flowers. Such a peduncle is called scape . It may bear a single flower as in Lotus , or a cluster of flowers as in Onion . In Agave , it is branched and the branches bear flowers.

4. In most plants, pedicel arise from the axil of a very small greenish, or brownish leaf-like structure on the peduncle, called bract . Bracts are highly reduced and modified leaves and their arrangement on the peduncle is the same as that of leaves on a vegetative shoot. When the peduncle is branched, the branches arise from the axils of similar bracts. Bracts differ from leaves in some respects, such as colour , size and shape. Besides these bracts, there may be one or two still further reduced bract-like structures on the pedicel, known as the bracteoles . Flowers having bracts and bracteoles are described as bracteate and brocteolate respectively. If these structures are absent, then the flowers are said to be ebracteate and ebracteolate .

5. Kinds of bracts Usually , flowers arise from the axils of bracts. But, in some cases, flowers may not be seen in the axils of bracts. For example, in pineapple ( Ananas comosus ), there are a number of bracts without flower buds in their axils, seen at the top of the inflorescence. Such bracts are said to be empty or sterile. The normal function of the bracts is the protection of the flower bud. But , in some flowers, the bract becomes brightly coloured to attract insects. Such coloured bracts are seen in Bougainvillea and Poinsettia .

6. In most plants, bracts are very small in size, and their leafy nature is not very clear . But , in some plants like Adhathoda and Justicia , they are exactly like foliage leaves in appearance. Such bracts are called leafy bracts . Sometimes , a bract of large size surrounds the entire inflorescence and protects the flowers when young. Such a bract is known as spathe , e.g.. Cocos nucifera . In grasses , bracts are known as glumes . In condensed inflorescence, as in sunflower, a number of bracts form an outer covering of the whole inflorescence. This covering is called are called involucral , and the bracts are called involucral bracts . At times, a number of bracteoles are found in a whorl on the pedicel immediately below the calyx (e.g. Hibiscus, cotton ). This whorl is called epicalyx .

7. Classification of inflorescence Based on the nature of the peduncle and the arrangement of flowers three main types of inflorescence can be recognized, namely Racemose Cymose Special.

8. A. Racemose inflorescence

9. Also called indefinite, or indeterminate inflorescence. In it, the main axis does not end in a flower and is capable of unlimited (indefinite or indeterminate) growth. Flowers are arranged on either side of it in acropetal succession. Sometimes, the main axis may give rise to a number of branches in acropetal succession and the flowers are arranged on these branches, in a similar manner. At times, the main axis may become very short and the flowers form a cluster with the older flowers at the periphery and the youngest one at centre. Racemose inflorescence is of several kinds. The commonest ones among them are raceme , panicle , corymb , spike , spadix , umbel , capitulum and catkin .

10. I. Raceme This is the simplest type of racemose inflorescence. It has a main peduncle on which a number of flowers are borne in acropetal succession. All the flowers are pedicellate , e.g , Glyricidia , Crotalaria .

11. Simple raceme - Crotalaria

12. II. Panicle Basically this is a branched raceme. The main peduncle gives rise to a number of secondary peduncles in acropetal succession and flowers arise on the secondar peduncles in acropetal succession. In yet other instances, secondary peduncles may branch and the flowers arise on the tertiary peduncles e.g., mango, teak .

13. Panicle-Mango

14. III. Corymb This is a type of raceme in which the peduncle is very short and it bears acropetally arranged pedicellate flowers. The pedicels of the flowers have different lengths. The oldest (basal) flowers have longest pedicels, and the youngest (terminal) ones have shortest pedicels. As a result, all the flowers of the inflorescence stand more or less at the same level, e.g.. Caesalpinia , Gynandropsis . In many cases, the main axis of the corymb elongates after flowering so that the corymb becomes a raceme.

15. Corymb - Caesalpinia

16. Caesalpinia Gynandropsis

17. IV. Spike This resembles a raceme except for the sessile nature of flowers. e.g., Achyranthes , Celosia . In some cases, the main axis of the spike is branched, and each branches resembles a spike. Such an inflorescence is called compound spike, e.g., Aerva . The small spikes found in grasses are called spikelets .

18. Spike- Achyranthes

19. Achyranthes , Celosia, Aerva

20. Other examples

21. v . Spadix This is the racemose inflorescence in which the peduncle is thick, fleshy and coloured . Flowers are small, sessile and unisexual and they sink into the depressions on the axis called spodices . The whole inflorescence is covered by a leathery bract called spathe . Generally, female flowers are basal and male flowers terminal in position. Spadix inflorescence is characteristic of monocots. e.g. Colocasia , Cocos , Areca, Musa, Amorphophallus . In Cocos and Areca the inflorescence is branched and the spathe is boat-like.

22. Spadix Spathe

23. Spadix Colocasia Cocos

24. Areca Musa

25. Amorphophallus

26. VI. Umbel This is the recemose inflorescence in which the main peduncle is short and bears at its tip a whorl of bracts. From the axil of each bract, arises a pedicellate flower. flowers are arranged in centripetal succession (older flowers towards the towards the margin and younger ones towards the centre). As in corymb , here also all the flowers are in the same level. But in corymb , the pedicels are different lengths and start from different length of the peduncle. In umbel, all the pedicles are of equal lengths and all of them start from the apex of the peduncle. The recemose nature of the inflorescence is evident from the centripetal arrangement of flowers. The main peduncle may be simple, or it may bear a cluster of flowers at the apex as in onion. In compound umbel, the main peduncle may give rise to secondary peduncles, and Pedicel pedicels start their tips in an umbellate manner as in coriander and carrot. The whorl of bracts below the cluster of pedicels forms an involucre .

27. Umbel - Biophytum

28. VII. Capitulum or head This racemose inflorescece is peculiar in that the main peduncle gets flattened out at the apex into a slightly convex platform-like structure, called receptacle . On the surface of the receptacle numerous small and sessile flowers are arranged in centripetal succession . At the base of the receptacle, there are numerous green and triangular bracts, which form an involucre . The individuals flowers on a head inflorescence are called florets . The florets arranged along the periphery of the receptacle are called ray florets and those arranged along the centre are called disc florets. Ray florets are usually zygomorphic and female, whereas disc florets are actinomorphic and bisexual. In some plants, such as Mimosa, Gomphrena , etc., the receptacle is not flat, but is a swollen and spherical structure and the flowers are arranged all over it. The whole inflorescence looks like a small badminton ball, and is known as a globose head.

29. Head or Capitulum - Sunflower

30. VIII. Catkin or amentum Catkin is a pendulous type of inflorescence, similar to spike, with sessile and unisexual flowers. The axis is thin and weak. e.g.. Acalypha , Morus , Artocarpus .

31. Catkin or amentum - Morus alba

32. Acalypha Artocarpus

33. B. Cymose inflorescence

34. Cymose inflorescence corresponds to the sympodial branching of the vegetative shoot. Here, the peduncle has only limited (determinate) growth, and it terminates in a flower. This terminal flower blocks the further growth of the peduncle. Subsequent flowers arise from the axils of the bracteoles of the first formed flowers. In racemose inflorescence, the bracteoles of the flowers do not bear flowers. But here, the bracteoles of the first formed flower become the bracts of the second set of flowers. Peduncle may have lateral branches which also end in flowers. In most cases, flowers are arranged in basipetal succession so that basal flowers are younger and terminal ones are older.

35. There are five main kinds of cymose inflorescence, namely Solitary cyme Simple cyme Dichasial or biparous cyme Monochasial or uniparous cyme Polychasial or multiparous cyme .

36. I. Solitary cyme This is the simplest type of cymose inflorescence in which there is only a single flower. The flower may be terminal on the primary axis, or axillary in position. The former is called terminal solitary cyme (e.g., Gossypium ), and the latter is called axillary solitary cyme (e.g., Hibiscus, Datura ).

37. Hibiscus

38. Gossypium

39. Datura

40. II. Simple cyme or cymule This is a cluster of three pedicellate flowers, one terminal and the others lateral and axillary (e.g., Jasminum ). In simple cyme , the main axis ends in a flower. This flower has two bracteoles and from the axil of each bracteole, a flower develops. Thus, the middle flower is the oldest one and it will open first, while the lateral ones are younger. The development of the flowers is centrifugal, i.e., from the centre to periphery. The arrangement is just the opposite of what is seen in racemose cluster, where the older flowers are at the periphery and the younger ones in the centre of the cluster.

41. Jasminum

42. III. Dichasial ( biparous ) cyme This corresponds to the dichotomous branching of vegetative shoots. In this case, the main axis ends in a flower. This flower has opposite bracteoles. A flower arises from the axil of each bracteole. Each of these flowers again has two bracteoles, which become the bracts of the third set of flowers. This results in the formation of a regular cluster developing in a symmetrical manner. The cluster can easily be divided into small groups of three flowers each. In each group the centrifugal development can be seen clearly. Dichasial cyme is found in Clerodendron , Ixora , Dianthus , etc.

43. Dichasial cyme - Clerodendron

44. Ixora

45. Dianthus

46. IV. Monochasial ( uniparous ) cyme This is the cymose inflorescence in which the peduncle ends in a single flower. It differs from dichasial cyme in that the main peduncle produces only a single secondary peduncle, instead of two. The secondary peduncle, in turn, produces a single tertiary peduncle. Of the two bracteoles of each flower only one is developed, while the other one is suppressed. When this suppression occurs only on one side, lateral branches and flowers appear on one side only. This type of is called monochasial helicoid cyme (e.g., Hamelia ). If branches and flowers develop alternately on both the sides of the pedencle , the inforescence is called monochasial scorpiod cyme (e.g., Heliotropium ). In helicoid cyme , all flowers are on the same side of the axis while in scorpioid cyme the flowers alternate on the axis.

47. Monochasial scorpioid cyme - Heliotropium inflorescence

48. The Monochasial helicoid cyme

49. V. Polychasial ( multiparous ) cyme This is the cyme in which more than two secondary and tertiary peduncles are produced (e.g., Calotropis , Mollugo .)

50. Calotropis Mollugo

51. Differences between racemose and cymose inflorescences Racemose inflorescence Cymose inflorescence 1. Peduncle has unlimited growth Peduncle has only limited growth 2. Peduncle does not end in a flower Peduncle ends in a flower. 3. Flowers develop in acropetal or centrifugal succession. Flowers develop in basipetal or centripetal succession

52. C. Special types of inflorescence

53. These are strictly neither racemose nor cymose . At the same time, they are modified forms of the cymose type. They include Hypanthodium Verticillaster Cyathium Thyrsus.

54. I. Hypanthodium This is the condense type of cymose inflorescence, more precisely a modifi-cation of the capitulum , characteristic of Ficus . Here, the peduncle forms a hollow, fleshy and flask-shaped receptacle. The receptacele has a terminal opening, called ostiole , for the entry of insect pollinators. The ostiole is guarded by numerous incurved hairs. Many small, sessile and unisexual flowers are arranged along the inner walls of the receptacle. The flowers include male female, and sterile flowers. Male flowers are seen near the ostiole , sterile flowers in the middle and the female flowers towards the base.

55. II. Verticillaster This is a compound inflorescence, consisting of two axillary opposite cymose inflorescences developing one on either side and meeting around the stem. It is typically found in plants with opposite leaves, and is characteristic of the family Labiatae (e.g., Leucas ). In this case, two clusters of sessile and opposite cymose inflorescences develop from the axis of opposite leaves as a dichasium . The subsequent development is of a monochasial scorpioid nature. During this, they bend round the stem and node and meet together. So, at each node, there is a cluster of flowers completely surrounding the stem.

56. Verticillaster-Leucas

57. III. Cyathium This is a highly condensed type of cymose inflorescence. It look a single flower, with an involucral cup, formed by the fusion of bracts. Peduncle is very much reduced. Terminally, it bears a single, highly reduced and central female flower, surrounded by five scorpioid cymes of reduced male flowers. Female flower is long-stalked and naked, and male flowers are short - stalked and naked and each of them is reduced to a stamen. Cyathium is often described as " cyme of cymes". It is the characteric inflorescence of the genera Euphorbia, e.g.. Acalypha , Poinsettia , etc.

58. Cyathium inflorescence-Euphorbia HABIT LONGITUDINAL SECTION

59. Poinsettia

60. IV. Thyrsus This is a mixed inflorescence, composed of a number of simple cymes arranged in a racemose manner. There is a main peduncle giving rise to secondary inflorescences on the axils of bracts in acropetal succession. Each secondary peduncle has flower two bracteoles From the axils of each bracteole a single flower arises. Thus each secondary peduncle forms a simple cyme of three flowers of which the central one is the oldest, e.g., Ocimum .

61. Ocimum