546 STRUCTURE AND LIFE HISTORIES 



plant, and upon the tomato were grafted three other 

 species — Solanum nigrum, Solatium 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- 



