Shoot Apex

The Shoot apex is also known as the terminal bud of plants that grows from 0.1-1.0 mm and consists of the apical meristem, developing leaves and the immediate surrounding leaf primordial. The shoot apex is present in both dicot and monocot plants.

1. Shoot Apex -Stem- Prepared by: Haider Ali Malik

2. EXTERNAL PLANT MORPHOLOGY

3.  A stem is an organ consisting of  An alternating system of nodes , the points at which leaves are attached  Internodes , the stem segments between nodes

4.  An axillary bud is a structure that has the potential to form a lateral shoot, or branch  An apical bud , or terminal bud, is located near the shoot tip and causes elongation of a young shoot  Apical dominance helps to maintain dormancy in most non-apical buds  Lenticels are structure that permit the passage of gas inward and outward.  Leaf scar are characteristic scar on stem axis made by leaf abscission.  Bud scales are small modified leaves for protection from desiccation.

5. Dormant shoot apex with its protective scales is a BUD.  Bud Scars are the scars left from the removal of bud.  Leaf primordium is an immature leaf of the shoot.  Intercalary meristem the portion of the internodes above the node . Made up of actively dividing cells responsible for the elongation of the monocot stem.

6. Fig. 35-12 This year’s growth (one year old) Apical bud Bud scale Axillary buds Leaf scar Bud scar Node One-year-old side branch formed Internode from axillary bud near shoot tip Leaf scar Stem Bud scar left by apical bud scales of previous winters Leaf scar Last year’s growth (two years old) Growth of two years ago (three years old)

7. Shoot Apex Organization

8.  The outer group consisting of one or more peripheral cell layer is known as the TUNICA . These cells divide anticlinally ( perpendicular to the surface of the shoot apex)  The CORPUS lies below the tunica and initially has a single layer of cells. Corpus cells divide anticlinally and periclinally (parallel to the surface of the shoot apex.) Shoot Apex organization

9.  A shoot apical meristem is a dome- shaped mass of dividing cells at the shoot tip  Leaves develop from leaf primordia along the sides of the apical meristem  Axillary buds develop from meristematic cells left at the bases of leaf primordia

10. Fig. 35-16 Shoot apical meristem Leaf primordia Young leaf Developing vascular strand Axillary bud meristems 0.25 mm

11.  Shoot apex- where new leaves and tissues of the stem arise  Models of tissue organization in the shoot apex A. Apical cell theory B. Histogen theory C. Tunica-corpus (most accepted in angiosperms)

12. *Pteridophytes- apical cell (1 initial) or apical initials – tetrahedral (pyramidal), base is directed towards the surface of the apex *Gymnosperms- surface meristem; (apical initials – periclinal) central mother cells

13.  Histogen theory (Hanstein, 1868)   1. dermatogen – outermost 2. plerome – central 3. periblem – between 1 and 2 Each develop from independent group of initials (histogens) Meristems are destined from the beginning to produce certain tissues

14. 1. 2. All cells have basically equal potential of differentiation One zone of apical meristem may contribute cells to another one

15.  Two regions: TUNICA and CORPUS  No constant relationship can be traced between the particular initials of the promeristem and the inner tissues of the shoot  2 regions can be distinguished by their plane of cell division

16.  Outermost layer  Surrounds the inner cell mass (corpus)  Anticlinal division  Enlarges in surface area  Layer: 1-9

17.  Inner cell mass  Divides in all directions  Enlarges in volume TYPES A. Usual – 1. CMC 2. rib meristem 3. peripheral B. Opuntia -- + cambium-like transition zone

18.  Central zone – (waiting meristem)- promeristem - corpus + portions of tunica - gives rise to:  Rib zone or pith rib meristem - below central zone; center location - becomes the pith  Peripheral zone or peripheral meristem - encircles the other zones - most meristematic (eumeristem) - densest protoplast and smallest dimensions - gives rise to leaf primordia,procambium, cortical ground tissue

19.  Initiated by periclinal divisions at the side of the apical meristem  Origin: tunica or corpus  Division  leaf buttress  Affects Periodic changes in shape of shoot apex BRANCHING  Where do branches originate?  Superficial layers --- exogenous  Axillary buds