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 asci (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 stylo gonidia 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 the sporophyte gives them origin. 

 The fact that they do not usually 

 germ ; na t e 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- 



