1898] NOTES AND COMMENTS 17 



water, from the suggestions as to improved methods of filtering out 

 the minutest organisms and all of them. Such methods are hardly 

 applicable to tow-nettings on a large scale such as interest most 

 naturalists — they are valuable to the laboratory man rather than tO' 

 the collector ; and though the naturalist ought to be both, he is 

 limited to being only one at a time. The point of most interest is 

 the open declaration against the assumption that supports the 

 Hensen method — viz., that No. 20 silk stops almost all organisms. 

 If one estimates the bulk of plankton on this basis there can only 

 be error, as the author points out. One might go further than he does 

 and insist that the filtering power of such silk varies very greatly 

 with its condition. For example, a new tow-net of No. 20 miller's 

 silk (the best procurable) stops comparatively few organisms. When 

 it has been used for a time it improves vastly as the meshes get 

 clogged with diatoms, Peridiniae, etc. If it be carefully washed 

 the net will then be at its best, and after reaching its optimum will 

 soon decline — wear out. This may be tested most easily by 

 pumping water into a net with another outside, or even with two 

 outside. Organisms, and these not the smallest only, will be found 

 in the outer bag even at the best of times. Similarly one may test 

 a new net against an older one by towing them at the same depths. 

 Estimates of plankton therefore by the Hensen method are bound to 

 be misleading, and, however carefully the error may be calculated 

 for this or that size of mesh, it is impossible to give the error 

 exactly, since the silk itself varies with use. 



Fermentation and Ferment Action 



In the recent issue of the Annals of Botany (Dec, 1897) Prof. 

 Eeynolds Green gives an account of some experiments with yeast. 

 They were made with the object of testing the statement of a 

 German observer, Dr Buchner, who claims to have extracted from 

 yeast a liquid having the power of setting up fermentation in sugar 

 solution. The importance of such a statement will be evident to 

 every student of botany, who, if properly brought up, realises the 

 great difference between the alcoholic fermentation induced by the 

 living yeast plant, and the ferment action set up by a series of 

 ferments or enzymes, which are merely formed substances, and can 

 be extracted from the plant or animal tissue, and made to perform 

 their peculiar functions quite apart from the living organism. Such 

 are the diastatic, peptic, and tryptic ferments which are universally 

 present where, in the first instance, starch or some forms of insoluble 

 carbohydrate food, and in the other two, proteid material has to be 

 brought into a soluble form like sugar, peptone or amide, before it 



