XI] STRUCTURE OF POLLEN 91 



in the sac, now enter into division, and each forms four 

 pollen-grains, either by division into two complete cells, 

 each of which then divides again (as occurs in most 

 Monocotyledons), or by the division of the nucleus into 

 two, followed by a repetition of the division before any 

 cell-walls are formed, as occurs in Dicotyledons generally. 



In either case the result is the same. Four pollen- 

 grains, each a complete cell with a thin cell-wall, proto- 

 plasm, and a large nucleus, lie in the swollen and rapidly 

 disorganising shell which is all that remains of the pollen 

 mother-cell. 



Just as differences occur in the number of pollen 

 mother-cells developed, so in that of the pollen-grains ; 

 but it is also true that whereas in most plants all the 

 pollen-grains soon separate and lie embedded in the 

 fluid formed by the disorganised surrounding cells and 

 remains of the mother-cells, there are a few cases where 

 the pollen-grains do not become free, but remain joined 

 in a tissue in each pollen-sac (most Orchids and Ascle- 

 piads), or only partially separate (many Mimosas and 

 Acacias, some Orchids, Fourcroya, Typha, Anona, Rho- 

 dodendron). Such groups of non-separated pollen-grains 

 are termed pollinia. 



The freed pollen-grains, living at the expense of the 

 fluid in the pollen-sac, now undergo two sets of changes 

 as they ripen. These concern first the cell-walls, which 

 thicken and become lamellated into an outer protective, 

 cuticularised wall (exine), and an inner thin cellulose 

 membrane (inline) \ become studded with the various 

 projections spikes, warts, comb-like ridges, networks and 

 other sculpturing which subserve distribution and ad- 

 hesion to insects and stigmas ; and provided with the 

 thin places which facilitate the exit of the pollen-tubes. 



These changes concern, secondly, the contents. The 



