114 PLANT PHYSIOLOGY 



they may be strongly marked. They are often called 

 " mutations " to distinguish them from the non-in- 

 heritable variations. If the plants showing them are 

 considerably better able to exist, they will rapidly crowd 

 out the less favorably constituted plants, and thus a 

 new species will replace the old. Under other environ- 

 mental conditions this new feature may be less favorable 

 and so the older form will persist. Thus we find plants 

 with all sorts of differences or what we call "species," 

 all over the world. Some plants have changed but little 

 apparently from their primitive structure, as they were 

 able to persist in that form under certain conditions, 

 while some of their descendants, it may be, have pro- 

 gressed far along the evolutionary line. Thus we find 

 the Vegetable Kingdom made up not only of the ends of 

 long evolutionary branches but also of stragglers that 

 have progressed only a very little way, and of those that 

 have grown further before branching out in some other 

 direction. It is this fact that enables us to attempt to 

 show the probable course of evolution (phylogeny) of the 

 Vegetable Kingdom in our arrangement of the plants now 

 existing. 



166. The conditions that favor reproduction have 

 been worked out for a good many plants, but are un- 

 known for the vast majority. It seems that those con- 

 ditions that favor continued vegetative growth, such as 

 an abundance of water and all foods, tend to delay or 

 prevent reproduction. On the other hand, there must 

 usually be a certain amount of food stuffs stored up. 

 If these can be prevented from accumulating, or can be 

 used up by promoting vegetative growth, reproduction 

 will be held back. In many cases, however, the repro- 

 ductive stage comes on in spite of all efforts to keep it 

 back, showing that not all the factors are known. 



