55^ 



PHANEROGAMS, 



four 'special' mother-cells of the pollen are formed (Fig. 375). Thus the mode 

 of development of the pollen-grains in Monocotyledons resembles that of the 

 microspores of Isoetes. It sometimes happens that only one of the two secondary 

 cells divides, and then only three pollen-grains are formed, two of them being of 

 normal size, the third considerably larger : or again, neither of the two cells may 

 divide, or only imperfectly, and this leads to modifications of the size and shape 



Fig. 376.-5 a young pollen-cell oiFunkia ovata; the thickenings which project outwardly are still small, in the 

 older pollen-cell C they are larger ; they are arranged in lines connected into a net-work. 



of the resulting pollen-grains. In other Monocotyledons {Asphodelus albus and 

 luteus) the development of the pollen proceeds in the manner to be described 

 below as being characteristic of Dicotyledons \ A second process is especially 

 characteristic of Dicotyledons, in which, after the division of the nucleus of the 

 mother-cell, the two secondary nuclei divide in planes at right angles to that of the 

 first division and to each other; the four nuclei are thus arranged, as it were, in 

 the corners of a tetrahedron. The protoplasm becomes then constricted into four 



Fig. 377.—^ pollen-sac oi Althaa rosea seen from the side, m the archesporium ; B transverse section of an anther- 

 lobe showing the two pollen-sacs, m the mother-cells of the pollen, in ^ still united into a tissue, in B already divided 

 each into four pollen-cells, n the tapetum of the pollen-sac. Each anther-lobe, consisting of two pollen-sacs, is here 

 borne on a long branch of the filament. 



lobes, each nucleus forming the centre of one of the lobes, by the ingrowth of 

 the thick wall of the mother-cell ; a simultaneous formation of cell-walls now takes 

 place in the planes of division, the walls being attached to the margin of the 

 ingrowths of the wall of the mother-cell, and thus the four masses of protoplasm 

 which have become rounded off during the division become four distinct cells 

 (Fig. 378, A-E). 



The mass of cellulose round each of the daughter-cells of the 



^ [This account of the development of the pollen is taken from Strasburger (Zellbildung und 

 Zelllheilung, 3rd edition, 1880.] 



