552 
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.— j9 a young pollen-cell oi Funkia 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 
luieus) 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. — A pollen-sac oi Althaa rosea seen from the side, vi the archesporium ; B transverse section of an anther- 
lobe showing the two pollen-sacs, m the mother-cells of the pollen, in A 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 
ZelUheilung, 3rd edition, 1880.] 
