OF HETEROGENESIS 207 



A careful and unbiassed consideration of the following facts will, 

 however, I think, make it plain that the evidence is overwhelmingly 

 against Chemotaxis and the Infection Hypothesis : — 



1. Chemotaxis can only be supposed to operate at short distances. 

 But such Diatoms as are found within the spaces are never to be 

 seen on the surface of the Duckweed. 



2. The Diatoms that are commonly met with on the surface of 

 the thallus (a comparatively large Navicula and a Cocconeis) are 

 never found within the sub-stomatal spaces or the epithelial cells. 



3. Chemotaxis implies a direct power of movement in response 

 to an attractive influence. But none of the Diatoms on the surface 

 of the leaf, within the spaces, or within the epithelial cells have 

 ever been seen to move. 



4. The Diatoms in the spaces are found intimately intermixed 

 with the Algoid cells, and generally in situations into which they 

 could not be supposed to have the power of penetrating. 



5. The Diatoms can often be seen to have replaced Algoid cells, 

 rather than to have pushed them aside. 



6. Finally, in places, the Algoid cells can be seen elongating 

 into the forms of the Diatoms, and at the same time changing from 

 a bright green to a brownish-yellow colour. 



Moreover, since making these observations on L. gibba and L. 

 minor, I have ascertained that similar transformations of some of 

 the fission products of the Chlorochytrium which infests L. tri- 

 sulca are also to be met with in that species of Duckweed. The 

 Diatoms found in this species have been almost always very small 

 and of the Navicula type — no Nitzschias having ever been seen in 

 association with the segmentation products of this particular 

 variety of Chlorochytrium, although the Duckweed bearing it has 

 been taken from one of the same ponds from which I have obtained 

 my supplies of L. minor and L. gibba. 



It is worthy of note in this connection that in Lemna trisulca 

 there are no stomata. The active algoid spores penetrate, as 

 F. Cohn showed, by boring between the epithelial cells into 

 subjacent spaces, where they increase and multiply in practically 

 closed cavities, and become also surrounded by a kind of capsule. 

 Subsequently these active spores make their way out through very 

 minute apertures which they themselves form, but in this species 

 of Duckweed there are no widely dilated stomata through which 

 in earlier or in later stages, should they attempt it, Diatoms would 

 be free to enter. 



