300 BOTANY PART T 



sexually produced descendants (cf. p. 313 ff.). Variations appearing 

 in single individuals will, unless they are of an absolutely dominating 

 character, become modified and ultimately lost by crossing with 

 ordinary individuals. In such a case sexual reproduction tends to 

 maintain the constancy of the species. In other cases, as when one 

 parent possesses new and dominant characters or when both parents 

 tend to vary in the same direction, the deviation from the ancestral 

 form may be maintained or increased by sexual reproduction. 



The great tendency to variation commonly exhibited by hybrids 

 (p. 313) illustrates how the equilibrium of the complex of properties 

 of a sexually produced individual is affected by divergent parental 

 tendencies. But, even as a result of ordinary fertilisation, not only 

 small and readily disappearing variations (fluctuating variations) but 

 sometimes more striking ones occur, in which the offspring differs so 

 strongly from the parents in characters, which can be inherited, that it 

 appears to be a new species or sub-species. Of such petites esptces of 

 Draba verna some two hundred are known. In such sudden variations 

 (the occurrence of which v. KOLLIKER, and with him KORSCHINSKV, 

 term heterogenesis, while DE VRIES more recently calls it mutation) 

 these authors seek the starting-points of the origin of new species. 

 This would occur when a particular species passes, from unknown 

 causes, into a period of mutation such as DE VRIES demonstrated 

 experimentally in Oenoihera Lamarckiana. KORSCHINSKY collected a 

 number of historical examples of heterogenesis, of which Chelidonium 

 laciniatum, which appeared in a garden at Heidelberg in 1590, and 

 Capsella Heegeri, SOLMS, which only recently appeared, will serve as 

 examples ( 105 ). 



The fluctuating variations which largely determine the valuable characters of 

 economic plants (e.g. the high percentage of sugar in the Sugar Beet) are in con- 

 trast to the mutations not fixed on inheritance. Careful and continued selection of 

 the varying progeny is thus necessary to maintain the required standard of the race. 



The experience of cultivators and the recent experimental work 

 on lower plants carried out by KLEBS show that different groupings 

 of the internal and external conditions of life favour reproduction 

 and ordinary growth respectively. In fact growth and reproduction 

 frequently though not always appear to be mutually exclusive. 



Vegetative Reproduction 



Vegetative reproduction, the .purely quantitative character of 

 which as a mere process of multiplication has been emphasised, exists 

 generally throughout the vegetable kingdom, and but few plants, e.g. 

 some of the Conifers and Palms, are altogether devoid of it. Mention 

 has already been made in considering artificial propagation that, 

 from the separate parts or single cells, or even from the naked 

 protoplasts (Siphoneae) of many plants, the regeneration of a new and 



