NATURE 



[May 6, 1897 



discharged in yellow light ; but Klebs decides against 

 this, urging that plants grown in air freed from carbon 

 dioxide behave in the same manner. He does not believe 

 that the small amount of assimilation, which can occur 

 by means of the carbon dioxide set free during respir- 

 ation, can account for the process. Still, it must be 

 admitted that this is an objection which is not devoid 

 of some weight; and the fact that plants grown in certain 

 mineral solutions onlyioxm zoospores during light, seems 

 to indicate that nutrition may yet be found to lie indirectly 

 at the bottom of the mattei". At any rate, a good deal 

 more analysis of the conditions is here necessary before 

 we can safely formulate any theory. Klebs himself goes 

 on to speak of the darkness itself as constituting a reiz 

 or stimulus ; but it is not very easy . to see how a 

 negative condition can be quite appropriately so con- 

 strued. May not the light, or at least the yellow con- 

 stituent rays, be regarded as exerting a tonic or inhibitory 

 effect, which must first be withdrawn before the already 

 existing tendency can manifest itself in action ? It is 

 clear that the same objection might be urged in the case 

 of some other so-called stimuli ; but it seems very de- 

 sirable to avoid any ambiguity of expression, especially 

 in a subject already so difficult, such as is involved in 

 the use of the word stimulus {reiz), both for an active 

 promoting cause and for the removal of a restraining 

 influence. 



It is extremely instructive to contrast the conditions 

 which excite the formation, in Vaucheria, of sexual or 

 asexual reproductive organs respectively. Whereas dark- 

 ness is advantageous in the case of the latter, the sexual 

 organs are only produced in the presence of fairly strong 

 light, which further must contain just those less re- 

 frangible (yellow) rays which inhibit the production of 

 zoospores. The action of the light is here two-fold. 

 Firstly it operates by promoting assimilation, and in 

 this capacity it can largely be dispensed with, provided 

 suitable carbohydrate food be supplied to the plant. 

 Secondly it acts as a direct stimulus, which initiates the 

 formation of the sexual organs, and in this capacity it 

 cannot be replaced. But when once the stimulus has 

 effected the inception of the sexual organs, they may 

 continue to develop in greatly reduced light, the degree 

 of maturity to which they finally attain being largely de- 

 termined by the initial duration of the stimulus. In 

 Vaucheria the oogonia require a stronger excitation than 

 that sufficing to produce the antheridia ; and consequently 

 it is possible, by regulating the illumination, to raise 

 plants bearing only male organs. 



Similarly, Klebs determined the corresponding special 

 conditions in the case of a considerable number of other 

 algiE. Several of these are of particular interest as 

 illustrating the individual vagaries and idiosyncrasies of 

 the different species, and also as forcibly emphasising 

 the danger of drawing general conclusions rom an 

 insufficiently wide area of facts. For example, Hydro- 

 dictyon can be induced, as a general rule, to readily 

 reproduce sexually or asexually at the will of the experi- 

 menter ; but it occasionally happens, as the consequence 

 of certain modes of cultivation, that it develops a very 

 pronounced tendency to form zoospores only, and under 

 these circumstances all the ordinary methods which are 

 commonly efficacious in producing gametes are futile. 

 NO. 1436, VOL. 56] 



The plant must first be broken of its tendency to form 

 zoospores ; and this can be done by keeping it at a high 

 temperature, and in the dark. This inclination to a par- 

 ticular form of reproduction is of some significance when 

 taken in connection with the difficulty, which is often 

 experienced in many fungi, of securing any but the non- 

 sexual form of reproduction ; but it is of still wider 

 interest as once more illustrating the fact that, although 

 external stimuli may evoke this or that form of response, 

 the actual form of the response itself is, after all, not so 

 much determined by the nature of the stimulus as by 

 the particular condition of the special protoplasmic 

 mechanism through which it operates. 



Another example may be quoted as illustrating the 

 difficulty of drawing any general conclusion from Klebs' 

 experiments at present. This is not meant by way of dis- 

 paragement, for his results are in the highest degree 

 useful as affording numerous exact data, even though they 

 hold, it may be, only for isolated individual species. Thus 

 two species of QLdogotiium were investigated, namely, 

 G£d. dipla7tdruin and QLd. capillar e. Both of these were 

 found growing in the water, often side by side ; and yet in 

 hardly a single respect does the stimulus, adequate to 

 provoke the formation of zoospores in the one species, pro- 

 duce a similar effect on the other. The chief differences 

 may be shortly summarised as follows. 



(i) In GI,d. diplajidrum a rise of temperature is one of 

 the most effective means (provided too great heat be 

 avoided), whereas in the case of CEd. capillarc it produces 

 absolutely no effect whatever. 



(2) In QLd. dipia?tdru>n a transference from running to 

 still water produces zoospores, whether in light or dark- 

 ness ; the diminution of oxygen apparently providing the 

 real stimulus here. In CEd. capillare, on the other hand, 

 the reaction only occurs in the darkness; the latter 

 condition being, in this case, essential to success. 



(3) In QLd. diplandrum light is absolutely without in- 

 fluence on the process, whilst in GLd. capillarc it possesses 

 a powerfully inhibitive action. That mere darkening is 

 not the proximate cause of the zoospore formation, is 

 proved by the fact that the process only begins after a 

 prolonged stay (two days) in the dark ; that is, probably, 

 the withdrawal of light allows some change to proceed 

 within the protoplasm, and that the effect of this is to 

 act ultimately as a stimulus to the formation of the 

 swarm-cells. 



If anything were needed to show how important is the 

 nature of the protoplasm in each individual instance when 

 considering the result which may follow on identical ex- 

 ternal stimuli, it would be hard to conceive of a better 

 example than that afforded by the behaviour of these two 

 species of (Edogonium. What the nature of the internal 

 mechanism may be, or how the stimuli actually affect it, 

 is absolutely obscure — as obscure, indeed, as are the re- 

 actions of the plant to gravity or to the directive influence 

 of light — so soon as we seek to penetrate beyond the 

 region of mere empirical fact. In the case before us Klebs 

 suggests that plasmolysis, and other disturbances of the 

 normal relations of the salts dissolved in the cell-sap, may be 

 the determining factor, but his arguments are not very con- 

 vincing, and, indeed, such an hypothesis recalls the rough 

 and ready " explanations " which used to be put forward 

 as solving the riddles of heliotropism and the like ; but 



