248 SCIENCE PROGRESS. 



air in the light, can be kept for months and years without 

 forming zoospores ; yet at any time zoospores can be ob- 

 tained by plunging it into water. 



The alga can be cultivated easily in the well-known 

 Knops' nutritive fluid of inorganic salts, and in o*2-0'5 per 

 cent, solutions grows well but forms no zoospores so long 

 as the salts are supplied ; yet at any time vigorous plants 

 removed to pure water at once develop zoospores. 



Cultures in water only, or in very dilute salt solutions — 

 o*i-o"2 per cent. — in the light, remain sterile : but darken- 

 ing them more or less completely at once induces zoospore 

 formation. 



If exclusion of light is combined with either of the two 

 foregoing methods the stimulus to zoospore-formation is 

 quickened.^ 



In these cases the first zoospores appear in less than 

 twenty-four hours, and go on forming for weeks. A most 

 interesting discussion follows as to the nature of the changes. 



This is supplemented by experiments on the effects of 

 temperature, various light-rays, osmosis, organic nutritive 

 materials, mechanical stimuli, etc, and even if the reader 

 does not accept all Klebs' few and cautious conclusions as 

 to the probable actions of the various factors, he can 

 scarcely escape two convictions, rv.?. : (i) that in such an 

 Alga, every factor of the environment produces its own 

 effect as it varies, and (2) Klebs has shown that strict ex- 

 perimental methods can be applied to the solution of the 

 problem as to what that effect results from. 



It is premature to generalise widely from the results of 

 this work, since Klebs promises us another book containing 

 his conclusions, this first instalment being confined almost 

 entirely to the detailed descriptions of the experiments. 

 Nevertheless one cannot avoid drawing comparisons be- 

 tween the effects of light, temperature, moisture, food- 

 materials, etc., in stimulating the production of asexual or 

 sexual organs in these Thallophytes, and the effects of the 

 same agents in forcing higher plants to propagate by 



^ The general truth of this has been confirmed by Miss Periz in the 

 laboratory in Cambridge. 



