ANGIOSPERMS. 



55^ 



forms the layer of fibrous cells which cause the dehiscence of the anther (Fig. 382, 

 Gft); to this we shall have to refer hereafter. 



The mother-cells of the pollen are at first large and their walls thin (Fig. 374, 

 A, sm); but these increase considerably in thickness, though generally not uniformly 

 (Figs. 375, 378, A), the thickening matter being usually distinctly stratified. In 

 many Monocotyledons the mother-cells now become completely separated, the 

 pollen-sac becomes broader, and the cells float singly or in connected groups in 

 a granular fluid which fills up its cavity, as is shown in Fig. 374, B, a phenomenon 



Fig. 3y4-—f uH^ia cordata ; A transverse section through a 

 young pollen-sac before the isolation of the mother-cells svt, 

 ep the tapetum which clothes the anther-lobe, 7u wall of the 

 pollen-sac ; B the anther-lobe after isolation of the mother-cells 

 sm; ep the tapetum (X 500). 



Fig. 375.— Mode of formation of the pollen of Fitukta 

 ovata (X 550). In VII the wall of the daughter-cell has 

 absorbed water till it has burst ; its protoplasm is forcing 

 itself out through the fissure, and is lying before it 

 rounded off into a spherical form. 



which calls strongly to mind the formation of the spores of Vascular Cryptogams. 

 In other cases, however, as for instance in many Dicotyledons [TropcBolum, Alihcea^ 

 &c.), the very thick-walled mother-cells do not become isolated; they completely 

 fill up the pollen-sac, but may become separated after the rupture of the anther- 

 wall in water. In most Monocotyledons the large central nucleus divides, and 

 two fresh nuclei make their appearance ; this is followed by the division of the 

 protoplasm, and by the simultaneous formation of a cell-wall in the plane of 

 division. The same process is repeated in each of the two cells, and thus the 



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