SECT, u PHYSIOLOGY 299 



surrounding medium) to induce them to produce at will either non- 

 sexual swarm-spores or sexual cells. In many plants unfavourable 

 external conditions apparently give the impetus to a sexual mode of 

 reproduction. The sexual product (zygospores of Algae, Phycomy- 

 cetes) seems better able than the vegetative germs (swarm-spores of 

 Algae, conidia) to remain a long time at rest, and so withstand the 

 disastrous effects of an unfavourable environment. No inference can 

 be drawn, however, from the function of the sexual germs in this 

 instance concerning 'the necessity for the existence of a sexual, in 

 addition to a vegetative, mode of reproduction ; for in other cases it 

 is the vegetative reproductive bodies, as, for example, the spores of 

 Ferns, which are especially equipped for a period of enforced rest ( 103 ). 

 What makes digenetic reproduction essentially different .from 

 monogenetic is the UNION OF THE SUBSTANCES OF THE PARENTS AND 



THE CONSEQUENT TRANSMISSION AND BLENDING OF THE PATERNAL 

 AND MATERNAL PROPERTIES. 



It is in this qualitative influence that the chief difference between 

 sexual and vegetative reproduction is shown. And this may be re- 

 garded as the special advantage of sexuality. BY VEGETATIVE REPRO- 

 DUCTION^ THE QUANTITATIVE MULTIPLICATION OF THE INDIVIDUAL IS 

 SECURED, WHILE BY SEXUAL REPRODUCTION A QUALITATIVE INFLUENCE 

 IS EXERTED. THE VEGETATIVELY PRODUCED PROGENY CONSIST OF 

 UNMIXED DESCENDANTS ; THE SEXUALLY PRODUCED OFFSPRING, ON 

 THE OTHER HAND, ARE THE RESULT OF A BLENDING OF THE PARENTS. " 



In vegetative multiplication the complex of properties unfolded 

 in the descendants does not as a rule differ from that possessed by 

 the parent form. By vegetative multiplication all the varieties and 

 races of cultivated plants, the characters of which do not come true 

 by seed, are maintained. Variation may, however, occur among the 

 vegetatively produced progeny. The occurrence of this may be 

 compared to bud- variation, in which a single bud on a tree may give 

 rise to a branch deviating in colour arid form from the type ; such a 

 bud-variation may, years after, return suddenly to the characters 

 of the typical form. In the same way variations arise among 

 vegetatively produced plants, a remarkable example of which, .accord- 

 ing to BEYERINCK, is afforded by the Bacteria, the reproduction of 

 which takes place exclusively by division into two ( 104 ). 



The sexually produced offspring, on the other hand, endowed 

 with the properties of the father, can never be identical with the 

 mother-plant, but possess the properties of both parents. When these 

 are divergent they frequently play very different parts in the 

 descendants, some (dominant) characters appearing conspicuously, 

 while others (recessive characters) become less marked or remain 

 completely latent. la this way the descendants do not exhibit a 

 uniform mean between the parents, but some may resemble the father, 

 others the mother. These relations determine the character of the 



