44 MARGARET C. FERGUSON 



LariciO) and Coker ('02) says that in Podocarpus " as in other 

 cases" no cellulose-wall is formed. The small cell cut off by 

 the third and last division of the apical cell persists as a perma- 

 nent feature of the mature pollen-grain. Its cytoplasm is dis- 

 tinctly differentiated from that of the tube-cell, but no cellulose- 

 wall has been observed in connection with this cell, its boundary 

 being marked by scarcely more than a condensation of its periph- 

 eral cytoplasm. 



The Mature Pollen-grain. During the development of the 

 male gametophyte the cytoplasm of the large cell gradually 

 increases in amount, the vacuoles becoming smaller from the 

 region of the nucleus outward, and finally disappearing alto- 

 gether. The pollen-grain has the same size, form, and, so far 

 as the wall is concerned, the same structure as the microspore 

 just prior to its germination. The thick, innermost, partial wall 

 described in connection with the microspore still persists as a 

 very prominent characteristic of the mature pollen-grain. With 

 the expansion of the wings, certain protoplasmic portions of the 

 microspore-cell are left with no support except the delicate endo- 

 spore ; it therefore seems probable that this broad, incomplete 

 wall extending along the back and down the sides of the pollen- 

 grain has been developed for the purpose of strengthening these 

 weakened points in the spore-wall, and as an additional support 

 to the dorsal side of the pollen-grain. 



But, while the wall of the mature pollen-grain is identical 

 with that of the microspore, the essential or protoplasmic part 

 of the spore has undergone marked changes, as we have 

 already seen. One or two deeply staining lines, more often 

 one than two in the mature pollen-grain, lie on the dorsal side 

 of the pollen-grain apparently imbedded in its innermost wall. 

 Extending from this wall at its middle point is a strongly convex 

 cell, the antheridial cell, with delicately reticulated cytoplasm 

 and a comparatively large nucleus. Just below and always in 

 contact with this cell is the nucleus of the tube-cell. The cyto- 

 plasm of the tube-cell is closely reticulated and slightly more 

 dense than that of the antheridial cell. Imbedded in its cyto- 

 plasm are numerous starch-grains. In this condition the pol- 

 len-grain of Pinus awaits pollination (figs. 64, 65, plate VI). 



