3 66 



FOURTH GROUP. SEED-PLANTS. 



FIG. 294. Mother-cells of the pollen of Cucurbita Pepo ; sg 

 the external common layers of the mother-cell in process of disso- 

 lution, sp the so-called special mother-cells consisting of masses 

 of layers of the mother-cell which surround the young pollen-grains, 

 and are also subsequently dissolved./^ the wall of the pollen-grain, 

 the spikes of which grow outwards and pierce through the special 

 mother-cell, v hemispherical deposits of cellulose on the wall of 

 the pollen-grain, from which the pollen-tubes are subsequently 

 formed,/ the protoplasm of the pollen-grain contracted ; the pre- 

 paration was obtained by making a section through an anther 

 which had lain some months in absolute alcohol. Magn. 550 times. 



(Fig. 292 v), the latter of which may divide and form a tissue of two or three cells, 

 but most commonly remains undivided. The vegetative cell is only separated from 



the large cell by a layer of proto- 

 plasm which is soon absorbed ; in 

 isolated cases only a more substantial 

 membrane, perhaps of cellulose, has 

 been observed. The pollen-tube is 

 formed as in the Gymnosperms from 

 the large cell, and sometimes the 

 vegetative cell or cells take no part in 

 this process, and only the contents 

 of the large cell pass into the tube. 

 But usually, as was said, the wall or 

 layer between the small vegetative 

 cell, the prothallium, and the large 

 cell is absorbed, and the former 

 becomes detached from the inner 

 wall of the pollen-grain and assumes 

 a peculiar fusiform or crescent-shaped 

 appearance (Fig. 292 77); this 

 proceeding we may consider to be a 

 retrogressive metamorphosis. The 

 free-floating vegetative cell may itself 

 divide, and the cells thus produced 

 also pass into the pollen-tube and 

 there disappear. The mode of 

 formation of vegetative cells in the 

 pollen-grain is the same therefore in 

 Angiosperms as in Gymnosperms, 

 but in Angiosperms these cells are 

 either not separated by firm walls of 

 cellulose from the cells which form 

 the tube, or they are rarely so separ- 

 ated, and this produces the. small 

 complications above described. 



The pollen-tube is a protuber- 

 ance of the intine which breaks 

 through the exine usually at definite 

 spots prepared beforehand ; there 

 are often several or even very many 

 such points of exit in a grain (Figs. 

 296 a, 297 0), and it is possible 

 therefore for as many pollen-tubes 

 to be formed from it; but usually one only developes vigorously and effects 

 fertilisation. Apart from the sculpture which has already been described on 

 the exine, the external form and the structure of the outer coat of the pollen - 



FIG. 295. Pollen of Thunbergia alata. I and // in concentrated 

 sulphuric acid. IV, V and VII in the same acid after solution of the 

 intine. /// in Schulze's solution, in optical transverse section. VI in 

 strong solution of potash, e the exine, * the intine. The fissures in the 

 exine are evidently due to subsequent internal differentiation. 



