420 VEGETABLE PHYSIOLOGY 
The yeast-plant gives us perhaps the simplest form of 
this organ. Any cell can play this part; its protoplasm 
divides into a number of pieces, frequently four, each of 
which becomes rounded off and clothed with a new cell- 
wall. After a time the four new cells are liberated by the 
breaking down of the original cell-wall. They are deve- 
loped in more highly differentiated plants in special cells or 
chambers named ascz (figs. 166 and 167), in very variable 
numbers, and are known as ascogonidia or ascospores. In 
other cases they are produced by abstriction from a cellular 
outgrowth of the thallus (fig. 168), and in these again the 
number produced from a single cell 
may vary within wide limits. These 
are generally called stylogonidia or 
stylospores. There is an almost in- 
finite variety of these bodies to be 
met with in different plants, but the 
variety affects only the conditions of 
their situation and does not indicate 
any difference in their own structure. 
They are unicellular bodies, or simple 
protoplasts, each clothed with a deli- 
cate cell-wall. 
These asexual cells are usually 
spoken of as gonidia when they arise 
upon a gametophyte, and as spores 
when thesporophyte gives them origin. 
Fic. 168, — StyLoconrpia 
or EHurotium, PRODUCED The fact that they do not usually 
BY ABSTRICTION FROM aaa Ei . 7 . 
een germinate till after a period of rest, 
though this is often not very pro- 
longed, suggests that they originated in consequence of the 
plant needing certain cells which should possess the power 
of passing through times of exposure to unfavourable condi- 
tions without destruction. Such unfavourable conditions 
would be likely to kill the more delicate vegetative repro- 
ductive bodies. This view is supported by the fact that 
many of the lower plants, particularly Yeast, do not pro- 
