no PLANT PHYSIOLOGY 



it was possible by appropriate means to affect both growth and reproduction. 

 In individual cases certainly the effective stimuli necessary were extremely 

 varied, and, when we take a general view of all the organisms studied by KLEBS, 

 we can only say that the development of reproductive organs in place of mere 

 vegetative growth is determined by alterations (by way of increase or decrease) 

 in the general vital conditions (temperature, light, moisture, oxygen, organic 

 or inorganic nutrients). It is not essential that a stimulus previously absent 

 should be applied to produce such a result ; it is quite sufficient that quantita- 

 tive alterations be made in stimuli already present (KLEBS, 1904, p. 487). 



Before inquiring more closely how the environment really acts we will 

 investigate, in a single instance, the various stimuli by which one and the same 

 process in a selected species can take place. Let us glance at the formation of 

 swarmspores in Vauchena repens. KLEBS (1904, p. 497) observed this to take 

 place under the -folio wing conditions : 



1. On transferring the filaments from more to less concentrated nutritive 

 solutions or to water. 



2. By transferring filaments from air to water. 



3. By transferring filaments from flowing to stagnant water. 



4. By reducing illumination, or best of all by darkening completely. 



5. By reducing temperature nearly to the minimum. 



6. By increasing the amount of salts present nearly to the maximum. 

 The same result, however, does not take place in the same way when these 



six different agencies are allowed to operate. It is possible, on the other hand, 

 to divide them into three series (KLEBS, 1904, p. 497) : 



(a) The external condition operates so as to cause a sudden change in the 

 first 24 hours, when thereafter normal growth ensues, as in cases 1-3, when 

 the filaments are transferred from air to stagnant water, nutritive solution, or 

 flowing water. 



(b) The external condition operates at once, but continues only as long as 

 sufficient nutriment is present. Thus zoospore formation begins, e. g. after 

 the filaments are darkened. After the spores are shed growth begins again, and 

 then the ends of the filaments are once more changed into sporangia and so on. 

 The whole process comes to an end when the filaments become exhausted or 

 when the nutriment gives out. 



(c) The external condition operates only after a certain time, as in cases 

 5 and 6. Lower temperatures and increase of salts operate on the cells in such 

 a way that, after a few days apparently, zoospore formation takes place spon- 

 taneously, continuing for weeks. 



It is sufficient for us to note that certain of these reactions are readily 

 explicable from the biological standpoint, i. e. that they are purposeful and 

 essential to the existence of the alga. Swarmspore formation, which results 

 from the application of the agencies i, 2 and 3, is the way in which the alga 

 adapts itself to the new life conditions ; the old cells can no longer live under 

 the new conditions ; the plants which arise from these swarmspores, however, 

 go on living under them for an unlimited time. 



A comparable case would be the dying off of the aerial leaves and the forma- 

 tion of aquatic leaves in an amphibious plant when transferred from land to 

 water. The fourth condition (darkness) never occurs as a possible state of 

 existence, and hence no adaptation to that condition can take place. The plant 

 is always making fresh efforts to reach a more suitable environment by means of 

 swarmspores. The two last agencies are more difficult to explain, but it would 

 take us too far to formulate any suggestions as to their significance. 



Still, the biological meaning of the external factors in zoospore formation 

 is not really at present under consideration, but the physiological aspect of the 



