

Physiology of Plants. 81 



of alternation of generations (see chap, v.); (b) in 

 studying the great variety of reproductive processes, 

 both sexual and asexual, which occur in the Algae and 

 Fungi; and (c) in investigating the nuclear changes 

 before and after the union of the sex-cells. The most 

 striking new departure has been the introduction of 

 experimental methods. 



From time to time there have been isolated experi- 

 mental observations on the physiological conditions of 

 sex and reproduction in plants. Thus we Ex eriments 

 have De Bary's case of starved fern pro- oifsex and 8 

 thallia which only produced antheridia, or Repro 

 the familiar case of the yeast plant, which usually multi- 

 plies by buds, but produces spores when starved. But 

 no connected series of experiments was ever undertaken 

 until Dr. G. Klebs took the subject in hand. His 

 work, published in 1896, is a fine instance of the success 

 which attends an adherence to scientific method. With 

 great care and patience he experimented with fifteen 

 genera of Algae ( Vaucheria, Hydrodictyon, &c. ) and two 

 genera of Fungi (Eurotium and Mucor), making sure in 

 each case that he had a pure culture to start with. His 

 aim was to discover whether external conditions deter- 

 mine the occurrence of the various forms of repro- 

 duction, and what these conditions are. The factors 

 investigated were nutrition, moisture, light, tempera- 

 ture, and chemical reagents ; and the general result is 

 a proof that certain external conditions determine the 

 occurrence of asexual reproduction (by zoospores), 

 while others as certainly evoke sexual reproduction (by 

 gametes). 



A single illustration may be given. In the case of 

 Vaucheria, zoospores are always formed when filaments 

 which have been kept moist for some days are soaked 

 in water, or when they are removed from a very dilute 

 nutrient solution and placed in pure water, or when 

 those which have been growing in water or in a very 

 dilute nutrient solution are placed in the dark. On the 

 other hand, if the filaments are placed in a 2-4-per-cent 

 solution of cane-sugar in bright light, sexual reproduc- 

 tion by gametes always occurs. 



(M523) Jf 



