266 PLANT PHYSIOLOOY 



food for itself, stimulates to sexual activity, as is shown by 

 the formation and activity of the sex-organs. This seems 

 to be universally true in nature. Given the need of food 

 and the means of manufacturing or obtaining it, the plant 

 will be so especially engaged in the processes of nutrition 

 that reproduction is not undertaken. But on the other 

 hand, given a suflficient supply of food stored in its own 

 body and diminished or suspended means of manufacturing 

 more food, the plant will produce for a time in still water 

 both zoospores and sexual elements, while in the dark the 

 formation of zoospores (not of sex-organs) will continue 

 for some length of time. 



If plants are healthy, certain conditions will invariably 

 induce them, whether they are old or young, to remain 

 sterile. Certain other conditions will induce them to form 

 bodies for their non-sexual reproduction, still other condi- 

 tions will induce them to reproduce sexually. Eeproduction 

 then must be regarded, like movement, as a reaction or 

 response to stimuli. But just as stimuli the same in kind 

 and degree induce one response or the opposite or none at 

 all, according to the kind of organism, so in reproduction, 

 a stimulus or a combination of stimuli which induces one 

 kind of plant to reproduce itself sexually may induce other 

 kinds to reproduce themselves non-sexually or not at all. 

 This may be illustrated by Klebs's studies of Hydrodictyon. 

 This plant, the water-net, lives in still or only slowly run- 

 ning water, under normal conditions floats, and is physio- 

 logically as well as mechanically a very sensitive organism. 

 It possesses two very distinct methods of reproduction, by 

 non-sexual zoospores, and by sexual motile spores or 

 gametes. Gametes and zoospores are formed in the ordi- 

 nary vegetative ceUs, not in specially differentiated organs. 

 Until these cells have attained a certain, though very 

 minute, size they cannot form reproductive bodies. Whether 

 there is a maximum size or not cannot be positively stated. 

 Since gametes and zoospores form in vegetative cells indis- 

 tinguishable from each other, and since all or some of the 

 cells of a net will form zoospores or gametes or neither, it is 

 obvious that each cell possesses in equal measure the ability 



