THE SEED PLANTS 301 
ae es its way through the central softer tissues of the style. It 
dc2s not make a passageway by forcing the tissue aside, but 
sy means of its own secretions (enzymes) it breaks down these 
tissues, and they doubtless furnish nourishment to the grow- 
ing pollen tube (fig. 230). When the tube reaches the cavity 
Fic. 230. Germinating 
pollen grains 
The pollen grains (g) have been 
deposited upon the stigma. The Fre. 231. Diagram of the ovule of an 
roughened surface of the stigma angiospermous plant, showing the parts 
is made by cell extensions, or of the ovule 
papilla (p). Pollen tubes (¢) grow ; 
tiom the grains through the tis- The outer integument (07); the inner integu- 
sue vs along the central canal (c) ment (i); the micropyle, which is the opening 
vAtil they reach the ovule. Only between the parts of the inner integument; 
‘a small part of the stigma and the pollen tube (pt), which has grown through 
style are shown in this cut the micropyle 
which contains the ovules, it turns across to the micropyle of 
the ovule. It then grows through the tissue at the tip of the 
ovule to the end of the embryo sac (fig. 251). In some cases 
(elm and walnut) the pollen tube grows down to the base of 
the ovule, then up through it, and finally reaches the egg. 
