546 STRUCTURE AND LIFE HISTORIES 
plant, and upon the tomato were grafted three other 
species Solanum nigrum, Solanum integrifolium, and 
Physalis alkekengi. Each species was apparently not in 
the least altered by this drastic change in the conditions 
of its life. 
2. In asexual reproduction by spores the situation is 
quite similar to that in vegetative propagation, but in 
certain cases there is abundant opportunity for the proto- 
plasm to become more or less altered during the compli- 
cated changes that accompany nuclear division. This is 
especially the case in the reduction divisions preceding 
spore-formation in the sporophytes of higher plants, espe- 
cially when the plant is a hybrid; and in spore-formation 
in the sporangia produced from the zygospore of some of 
the filamentous fungi, like Mucor Mucedo. In the latter 
case the nuclear divisions, some time preceding spore-pro- 
duction, result in separating out the female (+) and male 
( ) strains, so that the spores in a given sporangium are 
unlike as to sex some. being female (+), some male ( ). 
This will be discussed more fully in the next chapter. 
Such changes result merely in distributing the heritable 
units (genes) of the mother-cell unequally to the daughter- 
cells, but introducing nothing new; they may, however, 
result in the complete loss of one or more heritable units, 
or in the formation of a new one, not existent in the parent. 
In the latter two cases we recognize a mutation. No hard 
and fast line can be drawn between the various kinds of 
asexual reproduction; there are various degrees of transi- 
tion between reproduction by spores, gemmae, bulbs and 
tubers, and the artificially severed buds and scions used 
in grafting and "slipping." 
3. In sexual reproduction there intervene between par- 
