REVIEWS. 407 



colourless, and the protoplasm of each young pollen-cell 

 becomes aggregated on one side, and is connected by strings 

 of protoplasm with the cell-wall. 



The " Tapeten-zellen" — inner cells lining the young 

 anther cavity — now swell, pressing the next layers of cells, 

 and starch is found in the epidermal and outer layers of 

 the anther cavity. 



The walls of the tetrads then dissolve, and the young 

 pollen grains float free in the anther cavity. Fine granules 

 between the free pollen cells are derived from the late 

 swollen and dissolved cell- walls. These are coloured yellow 

 by iodine and Schnitzels solution. 



The walls of the pollen-grains become firm and double 

 contoured and are marked by radial striae, and the nucleus 

 becomes flattened. Then an increase of the protoplasm 

 takes place, and the wall of the pollen grain is found to 

 become rapidly developed in two directions : a centripetal 

 growth in thickness is accompanied by a centrifugal develop- 

 ment of spikes on the outer surface. The thickening layers 

 are deposited ou the aforesaid double-contoured and firm 

 cell-wall, and are less refractive and more capable of swell- 

 ing than it ; the spikes arise as small knobs on the outer 

 layer. The separate pollen grains now lie in a matrix, in 

 which protoplasmic strings run from one grain to another, 

 and these strings carry numerous " microsomes." The' 

 material for the latter appears to be derived from the late 

 tetrad walls and the now diffluescent tapete cells. 



These " microsomes" nourish the external parts of the 

 developing pollen grain, becoming aggregated at the surface, 

 and especially where the spikes are forming. The inner, 

 swollen thickening layers are soon seen to be traversed by 

 pores which abut on the outer layer between the spikes, 

 while the superficial lamellse of the outer layer become 

 differentiated as a special layer, continued over the rodlets 

 which cause the radial striation. 



As the pollen-grains approach maturity their nuclei divide, 

 and each obtains two. All the matter derived from the 

 " tapete cells," &c., passes between the young grains, and 

 even the fragments of the broken-up nuclei of the former 

 become used for the nutrition of the latter. As this occurs 

 the protoplasm of each pollen cell rapidly increases in 

 amount, and the vacuoles disappear. 



So far, complicated as is its structure, the pollen grain 

 is enveloped by the " extine" only ; the " intine" now forms 

 as an independent layer around the protoplasm, and becomes 

 swollen and protruded inwards at places. 



