ANGIOSPERMS. 
553 
tetrad now becomes differentiated into concentric systems of layers, and these are 
enveloped by layers which are common to the whole tetrad (Figs. 378 JE, 379). 
If the tetrads have lain for some time in water, the layers usually burst, and the 
protoplasmic contents of the young pollen-cells are forced out through the fissure, 
and become rounded off into a sphere (Figs. 375 VII; 378 i^, G). Soon after 
the conversion of the mother-cells of the pollen into a tetrad, each protoplasmic 
mass becomes clothed with a new cell-wall, at first very thin, which is continuous 
with the inner layers of the wall of the mother-cell, as is shown by its becoming 
detached from them when caused to contract by alcohol. This is the true cell-wall 
of the pollen, which now increases greatly in thickness, and becomes differentiated 
Fig. jii^.—AHhcea rosea; A—-E division of the mother-cells of tlie pollen into four; F and G a tetrad, the 
walls of whose special mother-cells have burst under the influence of water, and have allowed the protoplasmic body 
of the young pollen-cells to escape ; H a mature pollen-grain seen from without, magnified to the same extent 
(c/. Fig. II). 
into an outer cuticularised layer and an inner one of pure cellulose, the Extine and 
the Inline. The former becomes covered on the outside with spines (Fig. 379, pJi), 
warts (Fig. 376), ridges, combs, &c. ; while the latter frequently forms considerable 
thickenings which project inwards at particular spots (Fig. 379, v), and at a later 
period are employed to form the pollen-tube. During these processes the layers 
forming the envelope of the tetrad become slowly absorbed, their substance is 
converted into mucilage, and they at length entirely lose their form; their dis- 
organisation may commence either on the inner (as in Fig. 375, VII, x) or outer 
side (Fig. 379, sg) of the wall of the mother-cell. In consequence of this absorption 
