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. 
