250 SENESCENCE AND REJUVENESCENCE 



by the formation of gametes, experiment has demonstrated that this 

 life history is by no means fixed in its course. In the fungus 

 Saprolegnia mixta, for example, which occurs on the bodies of dead 

 insects in water, Klebs ('03, p. 41) has found that uninterrupted 

 vegetative growth may occur for an indefinite period in all good 

 nutritive solutions, provided they are kept fresh and do not under- 

 go alteration. On the other hand, a rapid and complete transforma- 

 tion of the vegetative form, the mycelium, into sporangia occurs 

 when a well-nourished mycelium is transferred from the nutritive 

 solution to pure water. Growth and vegetative reproduction, 

 together with continuous spore formation, occur in cultures nour- 

 ished on agar-albumin in flowing water. When mycelium grown 

 on gelatin-meat extract is transferred to water and allowed to 

 continue its growth on dead insects, growth and vegetative repro- 

 duction are followed, first, by formation of spores, and later by 

 gamete formation. In water containing fibrin or syntonin growth 

 and vegetative reproduction, formation of spores and of gametes 

 occur together on different parts of the plant. In a weak solution 

 of haemoglobin, growth and vegetative reproduction are followed by 

 formation of gametes and later by formation of spores. 



Another example is the alga Vaucheria repens. According to 

 Klebs ('04, p. 497), the following conditions induce zoospore forma- 

 tion: decrease of the salt-content of the medium to a point near 

 the minimum by transference from more to less concentrated solu- 

 tions, or to water; increase of moisture by transference from air to 

 water; decrease of the oxygen content of the medium by trans- 

 ference from flowing to standing water; decrease of light intensity, 

 even to darkness; lowering of temperature to near the minimum; 

 increase of the salt-content to near the maximum. 



Klebs believes that external conditions produce their effects on 

 organisms by acting upon a complex of internal conditions, and 

 he attempts to interpret his experimental results on this basis, 

 pointing out that many of the conditions which induce spore forma- 

 tion decrease or inhibit growth, i.e., vegetative reproduction. In 

 this way, as he believes, a higher concentration of organic substance 

 is attained in the plant, and this favors spore formation and a still 

 higher concentration, game tic reproduction. Apparently, for Klebs, 



