198 PERIODICITY OF GROWTH 



In FUNGI a deficiency of food often excites the formation of reproductive organs, 

 and the same also applies to the spore-formation of bacteria and yeast and to the 

 production of sporangia by Myxomycetes. A similar stimulus is required for 

 the development of the zoospores and oogonia of Saprolegnia^ of the zygotes of 

 Basidiobolus ranarum, and of the fructification of Coprinus stercorarius. The utility 

 of this phenomenon is obvious, but a different kind of action must be exercised 

 when certain modes of reproduction are excited in the presence of an abundance 

 of food. 



In Penicillium and Mucor it is the passage of certain hyphae into the air 

 which excites the formation of sporangia, which may be suppressed by keeping 

 the mycelium submerged in a nutrient solution. Similar relationships probably 

 exist in the case of parasitic fungi, and it might be possible under appropriate 

 conditions to grow such fungi entirely vegetatively, or to reverse the normal order 

 of appearance of sexual and asexual reproductive organs 1 . This is especially 

 applicable to those metoxenous fungi which complete their life-cycle on different 

 hosts 2 . 



ALGAE. Klebs 3 has shown that the alternation of generations in Vaucheria, 

 Spirogyra, Hydrodictyon^ Protosiphon, and other algae is entirely dependent upon 

 the external conditions, which determine whether growth shall be entirely vegetative, 

 or whether sexual or asexual reproductive organs shall be produced. The con- 

 ditions required differ in different cases, and often involve more than one factor. 

 Stronger illumination is, however, required for sexual reproduction than for the 

 formation of zoospores in Vaucheria^ or for the vegetative growth of Spirogyra. 

 On the contrary, darkness excites the production of sexual organs in Protosiphon, 

 and frequently a mere change of conditions often acts as a formative stimulus. 

 Vaucheria, for instance, produces zoospores when transferred from moist earth to 

 water. The existent conditions are of great importance, and thus Spirogyra, 

 Oedogonium, &c., form no sexual organs in a 0-2 per cent, nutrient solution, but 

 do so in water when all the required conditions are satisfied. A similar suppression 

 takes place so long as the water is kept in active movement, and although in this 

 case vegetation is especially vigorous, it does not follow that a retardation of growth 

 will induce the development of sexual organs. 



In ferns and mosses the formation of reproductive organs is influenced by 

 various external factors. 



In flowering plants the formation of flowers may be suppressed by darkness, 

 high temperatures, and other agencies. In fact the general rule is that vegetative 

 growth is possible within wider limits than is the formation of reproductive organs. 



In nature, therefore, the reproduction of algae and fungi is always 

 induced by appropriate changes in the external conditions. In fungi the 

 change is mainly the result of the plant's own activity, as for example when 



1 Cf. Klebs, Biol. Centralbl., 1899, Bd - XIX > P- 2I 4- 

 3 See also Klebahn, Bot. Ztg., 1898, Ref., p. 156. 



3 Klebs, Biol. Centralbl., 1899, Bd. xix, p. 209; Bedingungen d. Fortpflanzung, 1896; Ueber 

 einige Probleme d. Physiol. d. Fortpflanzung, 1895. 



