618 J. G. GRENPELL. 



motion. Under these circumstances the pseudopodia serve three 

 purposes: 1. Protection. 2. Means of attachment. 3. Floats. 



1. Protection. — The pseudopodia act in the ordinary way 

 as defensive spines. I have often seen large predatory In- 

 fusoria knocking these about, and absolutely unable to touch 

 them. The ordinary isolated diatoms can creep into mud or 

 debris out of harm's way. To these the stiff pseudopodia 

 would be quite useless. 



2. Means of Attachment. — At Heytesbury I found the 

 diatoms in running water, especially amongst filamentous 

 weeds. Here the use of the pseudopodia is quite obvious. 



3. Floats. — In the Botanical Gardens they are all over the 

 still waters, and no doubt the large extra surface given by 

 these pseudopodia helps to keep them floating. The remark- 

 able pelagic diatoms Chsetoceros also have long processes, but 

 of a different kind : that is to say much coarser, and obviously 

 forming part of the siliceous skeleton. 



The next question is the substance of which these pseudopodia 

 are made. I think the facts point conclusively to that sub- 

 stance being protoplasm. They are destroyed by nitric acid, 

 while those of Chaetoceros are not. All the finer parts are at 

 once destroyed by roasting at the lowest red heat. The thick 

 connecting bands and the thickened bases of the pseudopodia 

 will stand a low red heat occasionally, while they, too, are 

 entirely destroyed by a strong heat. 



They stain readily with Kleinenberg's hsematoxylin. With 

 Schultze's solution they give no cellulose reaction, nor with 

 iodine and sulphuric acid. Boracic carmine, which does not 

 stain cellulose, stains the bases of the pseudopodia strongly, 

 and just the same colour as the cell-membrane — the fine part 

 slightly. It is quite probable that some kind of cuticle is 

 secreted by the protoplasm in contact with the water, and that 

 this gives to the bases and connecting bands their resisting 

 power at a low red heat. The bases and cell-membrane always 

 behave in nearly the same way to stains. Two other proteid 

 stains, said not to stain cellulose, were tried. Picro-nigrosine 

 stained the diatom bodies well, the bases fairly, and also the 



