248 



BOTANY OF THE LIVING PLANT 



by walls perpendicular to the surface, so that the layer maintains its identity. 

 It develops into the epidermis (i,). The young stamen soon shows four 

 angular projections, which represent the four pollen-sacs. They project 

 owing [to ^the active growth and division of the hypodermal cells (ii. Hi.). 

 In the earliest state the hypoderma also appears as a single layer all round 

 (ii.) ; but as it grows older certain of its cells enlarge at the angles of the 

 section, and divide by walls parallel to the surface. Outer parietal cells are 



thus cut off from inner cells which are sporo- 

 genous, and give rise ultimately to the pollen 

 (ii. iii.). The outer cells undergo further 

 division by walls parallel to the first, forming 

 usually three cells each (iv.). Of these the 

 outermost provide, after further growth and 

 division, the fibrous layer ; while the inner- 

 most, which adjoins the sporogenous cells, 

 forms part of the nutritive tissue known as 

 the tapetum. The sporogenous cells them- 

 selves, which are shaded in Fig. 195, ii.-iv., 

 are easily distinguished not only by their 

 origin, but also by their dense protoplasmic 

 contents. The steps thus described are found 

 to be very constant in the anthers of Flower- 

 ing Plants. They provide material for com- 

 parison with other sporangia. There is some 

 latitude, however, in the number of the 

 divisions, and the figures show this even in 

 the single case of Chrysanthemum. Where 

 the stamen is large and its walls thick, as in 

 the Lily, more numerous divisions of the 

 hypodermal cells may provide a sporangial 

 wall thickened by extra layers. 



As the pollen-sac develops, the layer next 

 below the epidermis first acquires a store of 

 starch (Fig. 197) which is converted later into 

 the thickening of the walls characteristic of 

 the fibrous layer (Fig. 193). Within this 

 lie the featureless cells of the intermediate 

 layer. The group of sporogenous cells in- 

 creases in size, and often also in number of 

 cells by division. Each of these cells is called a pollen-mother-cell. The group 

 of them occupies the centre of the projecting sporangium, and is invested by 

 the continuous sheath of the tapetum (shaded in Figs. 196, 197). This appears 

 as a single layer of large cells with very thin walls and granular contents. 

 It is the result of development of the cells on all sides adjoining the 

 sporogenous group. Fragmentation of the nucleus frequently happens as 

 its cells grow old. Its function is to nourish the developing pollen. As the 

 grains mature the cells of the tapetum gradually collapse, their substance 

 being absorbed by the pollen. In the mature pollen-sac only vestiges of 

 them remain lining the sac internally (Fig. 193). 



FIG. 195. 



Successive stages in the early develop- 

 ment of the anther in Chrysanthemum. 

 (After Warming.) ( x 300.) For de- 

 tails, see text. 



