280 PHARMACEUTICAL BOTANY 
vascular bundle in the connective. Then, along four longitudinal 
tracts, rows of cells remain undivided or only divided slowly as 
they increase in size and around them cells divide and redivide 
to form the future endothecial and covering tissue to the four 
sporangia. Next, the four sporangial tracts of undivided cells cut 
off from their outer surfaces a layer of enveloping cells, the tapetum. 
This consists of richly protoplasmic cells that form a covering to 
the spore mother-cells within. Each spore mother-cell undergoes 
division and redivision into four spore daughter-cells, at the same 
time that reduction in the chromatin substance takes place in 
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Fic. 196.—Cross-section of a mature lily anther. The pairs of pollen chambers 
unite to form two pollen sacs, filled with pollen grains; s, modified epidermal cells 
at line of splitting. (From a Text-book of Botany by Coulter, Barnes, and Cowles. Copy- 
right by the American Book Company, Publishers.) 
these cells. Thus originate fetrads (groups of four) of spore 
daughter-cells inside the spore mother-cell wall. These continue 
to enlarge, press against the mother-cell wall, which becomes 
converted into mucilage, and each of the tetrad cells becomes in 
time a mature microspore or pollen grain. 
During this time the entire anther is growing in size, the cells 
of the endothecium in one or more layers becomes thickened by 
lignin deposits to form a mechanical endothecium; the tapetum 
gradually breaks down and appears only, at length, as an irreg- 
ular layer around the maturing pollen cells. When the anther 
is finally ripe the partition between each pair of microsporangia 
becomes narrowed, flattened and ruptured and thus numerous 
