GENERAL 199 



the consumption of food or the penetration of hyphae into another medium 

 affords the required stimulus. In algae, however, the stimulation is usually 

 due to such factors as light, water, or temperature, and a regular alternation 

 of generations would be maintained by a constant periodicity of the climatic 

 factors. 



In the case of flowering plants, ferns, and mosses, however, even 

 under constant external conditions, the progress of development would 

 ultimately lead to reproduction by sexual or asexual means, or by both. 

 Here the altered conditions which cause the equipotential meristem cells to 

 develop in a special direction are of internal origin, whereas in the algae 

 and fungi mentioned changes in the external conditions are required for 

 this purpose. Here also rhythmical alternation of generations may be 

 possible without any change in the external conditions, especially in the 

 case of the more highly organized Thallophyta. The fact that under certain 

 conditions such organisms as Vaucheria only grow vegetatively affords no 

 evidence to the contrary, for the same would apply to those flowering plants 

 which produce no flowers under certain conditions, and in which therefore 

 the change to normal conditions excites the formation of flowers. 



The potential powers of a plant are never exercised to their full extent, 

 except by some change in the external conditions, but it is not always the 

 case that, as in fungi, algae, and bacteria, only vegetative growth and only 

 vegetative reproduction take place under constant optimal conditions. Yeast, 

 however, multiplies rapidly during fermentation without ever forming spores, 

 and Klebs 1 was able to cultivate Saprolegnia for 2| years by purely 

 vegetative means. Many mosses and flowering plants have propagated 

 themselves for long periods of time by purely vegetative means, both in 

 nature and under cultivation 2 . 



Vegetative reproduction has therefore no weakening effect, and possibly 

 all plants could reproduce themselves for an indefinite length of time with- 

 out the formation of special reproductive organs 3 , although these are none 

 the less of importance to secure distribution and maintenance under 

 natural conditions 4 . Vegetative reproduction involves a constant repetition 



1 Klebs, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, pp. 151, 158. Although an organism may 

 grow indefinitely under constant conditions, a change may be of advantage, and may be necessary for 

 the attainment of optimal development. 



2 Cf. Mobius, Beitrage z. Lehre v. d. Fortpflanzung, 1897, p. 23. 



3 Cf. Klebs, 1. c., p. 179. Klebs (1. c., p. 161) has also shown that Maupas' experiments do 

 not prove the necessity of conjugation in infusoria. 



4 Klebs, 1. c., p. 179; Mobius, Beitrage z. Lehre v. d. Fortpflanzung, 1897. A plant which 

 under special conditions forms only male or only female flowers does so because certain 

 potential activities are suppressed and others excited or allowed to find expression. In unisexual 

 plants, however, the sex is already determined in the seed and cannot be altered, for it is not certain 

 whether particular conditions induce a predominant formation of one sex (Heyer, Ber. a. d. physiol. 

 Laborat. u. der Versuchsanstalt des landw. Inst. zu Halle, 1884, Bd. I, p. 43; Fisch, Ber. d. Bot. 

 Ges., 1887, p. 136; Molliard, Rev. ge*n. d. Bot., 1898, T. x, p. 324; Strasburger, Biol. Centralbl., 

 1900, Bd. xx, p. 722). 



