206 MISCELLANEOUS EXAMPLES 



appears utterly irreconcilable with the infection hypothesis, we 

 seem unavoidably driven to the conclusion, which is so congruous 

 with all the facts, that the Diatoms in question are heterogenetic 

 products, actually produced by the transformation of the cells of the 

 Alga — alike in the sub-stomatal spaces and in the epidermal cells. 



I include the epidermal cells in this statement because almost all 

 that has been said against the infection hypothesis and in favour of 

 the transformation hypothesis, as accounting for the presence of 

 the Diatoms in the sub-stomatal spaces, holds good also in regard 

 to their presence in the epidermal cells. In one respect the argu- 

 ment is even stronger in its application to them, since there is much 

 evidence to show that Diatoms are only found in those epidermal 

 cells which are or have been tenanted by the Alga, and such 

 infected cells never constitute more than the smallest fraction per 

 cent, of those existing on the whole upper surface of a leaf. 



A further point of extreme importance is to be found in the very 

 great differences in the size and shape of the Diatoms, according 

 as they originate from the small or the larger algoid fission products. 

 Yet these variations, for which no other contributory cause is 

 apparent, are so great that botanists, unaware of the origin of the 

 Diatoms, and finding them in the Chlorochytrium spaces, would 

 almost certainly regard some of them as belonging to different 

 species of the same genus, and others even as representatives of 

 distinct genera. This, however, is a subject which must be left for 

 future investigation. 



It was suggested to me by a distinguished botanist to whom I 

 showed some of the specimens of Duckweed, containing in their 

 sub-stomatal spaces and epithelial cells mixtures, in various propor- 

 tions, of Chlorochytrium segments and Diatoms, that their 

 association might be explained by the Infection Hypothesis, backed 

 by the assumption that Chemotaxis had been in operation — ^which, 

 in this case, would mean that the physico-chemical processes 

 associated with the growth and multiplication of the Algae within 

 the spaces were capable of giving rise to products exercising an 

 attractive influence upon the Diatoms. 



It was not pretended that there was any direct evidence in favour 

 of this assumption ; it was advanced as a possible explanation, and 

 merely to stave off the conclusion, otherwise inevitable, that the 

 Diatoms had been produced by the transformation of the cells of 

 the Alga. 



