482 HETEROMORPHISM AND ALTERNATION OF GENERATIONS. 



tubular Alga, Vaucheria sessilis (belonging to the family of the Siphonacese) will 

 serve as an admirable example of what we mean. This plant reproduces sexually 

 (of. pp. 57, 58, and figs. 204 5 and 204 6 , p. 53) by means of oogonia and antheridia of 

 simple character; it also propagates itself by means of large asexual zoospores 

 which it liberates from the tips of its tubular filaments (cf. vol. i. pp. 23, 24, and 

 Plate I., figs. a-d). But these two classes of reproduction do not occur simul- 

 taneously upon one and the same plant; but rather, so it was till lately supposed, 

 upon generations which alternated with one another either regularly or irregularly. 

 Sometimes the Vaucheria-pl&nt arising from a zoospore bore sexual organs, and 

 from the fertilized egg-cell arose a non-sexual plant which gave rise to zoospores 

 again; or a series of asexual generations followed one another, the series being 

 terminated by a sexual generation, the fertilized egg-cells of which entered on a 

 resting stage. 



The meaning of this supposed alternation of generations in Vaucheria has 

 recently been cleared up by Klebs in a series of very interesting culture-experi- 

 ments. Without describing these in detail we may briefly indicate some of Klebs's 

 results. If a number of young Vaucheria-pl&nts be cultivated, three possibilities 

 are open; the plants may produce sexual organs; they maybe reproduced asexually 

 by zoospores; or, finally, they may remain perfectly sterile. Klebs found that by 

 appropriate treatment of plants, he could bring about any of these possibilities at 

 will. Young plants placed in a 2-4 per cent sugar solution, and kept in the light at 

 a temperature not falling below 3 C., invariably produced sexual organs in the 

 course of some ten days. Other plants, which had been grown in a dilute nutrient 

 solution of food-salts in the light for a short time, were removed to water and placed 

 in the dark. These plants soon gave rise to enormous quantities of zoospores; in time 

 these zoospores germinated, and the resulting plants in their turn produced fresh 

 zoospores, and so on. The third condition, that of sterility, was obtained by keeping 

 plants in strong sugar solution (10 per cent), and also by other methods. More than 

 this, the same plants were caused to alter their mode of reproduction by varying 

 the conditions; in this way it was possible to cause them at one time to produce 

 zoospores and at another sexual organs. This brief summary is sufficient to show 

 that a given Vaucheria-pl&nt has no inherent tendency to reproduce asexually in 

 preference to sexually, or conversely; and that its manner of reproduction (or its 

 abstention from reproduction) depends on the conditions which prevail outside the 

 plant. Thus, in Vaucheria, no true alternation of generations prevails in the sense 

 in which it does in Mosses and Ferns, and every generation is potentially both a 

 sexual and an asexual generation. It is the external conditions which call forth 

 the one or the other. 



In a great many other Thallophytes the same is no doubt true, though exact 

 experiments have yet to be performed on the majority of them. We know it to 

 be so in Botrydium and in the Water-net (Hydrodictyon) and in others. The 

 Water-net (figured on p. 24) propagates asexually by the contents of its cells 

 breaking up into very numerous (7000-20,000) swarm-spores (thallidia) which 



