4^4 



PHANEROGAMS. 



does not take place till a much later period. The mother-cells of the pollen are at 

 first large and their walls thin (Fig. 343, A, sm) ; but these increase considerably in 

 thickness, though generally not uniformly (Figs. 344, 347, 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. 343, ^, a phenomenon which calls strongly to mind the formation 

 of the spores of Vascular Cryptogams. In other cases, however, as for instance 

 in many Dicotyledons (Tropaeolum, Althaea, &c.), the very thick-walled mother-cells 

 do not become isolated ; they completely fill up the pollen- sac, but are usually found 

 separated after the rupture of the anther-wall in water. With the thickening of 

 the cell-wall is connected a rounding-off of the protoplasm, the large central nucleus 

 of which is absorbed when the preparation is commencing for the formation of the 

 pollen-cells. Instead of the nucleus which has disappeared by absorption, either two 



Fig. 348.— Mother-cell of the pollen ofCucitrbiia Pepo ; S2 the outer common layers of the mother-cell in the ac 

 of being absorbed ; sp the so-called ' special mother-cells,' consisting of masses of layers of the mother-cell which 

 surround the young pollen-cells ; they also are afterwards absorbed ; ph the wall of the pollen-cell ; its spines grow 

 outwards and penetrate the special mother-cell ; v hemispherical deposition of cellulose on the inside cf the pollen 

 cell-wall, from which the pollen-tube is afterwards formed ; / the protoplasm contracted (X550). (The preparation 

 was obtained by making a section of an anther which had lain for some months in absolute alcohol.) 



fresh nuclei first of all make their appearance and undergo an immediate simulta- 

 neous bipartition (as represented in Fig. 344, /, //), or these two are again absorbed 

 and four nuclei are formed in their place, followed by the simultaneous division 

 of the cell into four. These cases have been observed especially in Liliacese 

 among Monocotyledons ; but a third process is especially characteristic of Dicoty- 

 ledons, in which, immediately after the absorption of the nuclei of the mother-cell, 

 four fresh nuclei are formed simultaneously, which arrange themselves at diff'erent 

 points of a plane or in the corners of a tetrahedron, the protoplasm becoming then 

 constricted into four lobes, each nucleus forming the centre of one of the lobes. 

 During this constriction the thick wall of the mother-cell grows inwards, following 

 the constriction of the protoplasm, until at length the four lumps of protoplasm 

 which have become rounded off" during the division lie quite distinct in four cavities 

 of the mother-cell (Fig. 347, A-E). The mass of cellulose now becomes diff'er- 

 entiated round each of the daughter-cells of the tetrahedron into concentric systems 



