394 NATURAL SCIENCE [December 



fiuence over a limited area of protoplasm, and ruler and ruled may 

 together be mentally mapped off as a unit. The body of Vaucheria 

 can therefore be described as consisting of a number of units — the 

 energids — enclosed within a single cell wall. 



Quite lately another small work dealing with these matters has 

 been written by Dr Adolph Hansen, Professor of Botany at the 

 University of Giessen. 1 



That part of the little book which deals with the ' Geschichte ' 

 is most carefully and judiciously written, and it at the same time 

 has a life and vigour in its sentences that fire our enthusiasm for 

 the subject. When we turn to the latter pages of the pamphlet in 

 which the ' Kritik ' is embodied, it awakens very mixed feelings 

 within us. It sets us thinking, which is a good thing, but nowhere 

 does it bring conviction with it. Where the views are most de- 

 finitely stated we feel the greatest doubt, and where the arguments 

 should be the most irresistible we are the least convinced. 



Glancing at what is written on pp. 50-58, we see that the 

 zoological definition, if I may so term it, of a cell is taken as the 

 starting-point of the argument. The zoologist {e.g. Professor Oscar 

 Hertwig) defines a cell as a little mass of protoplasm that includes 

 a nucleus within its substance. 



From this point of view Hansen maintains that the general 

 assertion ' that plants are composed of cells ' is untenable, since the 

 larger mass of them is built up of cell-walls ; moreover, whilst a 

 naked swarmspore may be correctly named a cell, it ceases to be 

 such immediately that it forms a wall around its surface. 



In considering these statements we must first ask whether it is 

 really a general assertion that ' plants are composed of cells.' I do 

 not think so. I myself, and all whom I have ever heard, have 

 always, both in animal and plant histology, stated that the plant or the 

 animal, as the case may be, consists of cells and the products of 

 cells. Our author meets this qualification in part, perhaps, by saying 

 that those who assert that plants consist of cells (in the zoological 

 sense) approach the difficulties in the above cases by regarding the 

 membrane as ' secondary ' or ' unessential,' and in that case he goes 

 on to argue how are we to look upon Caulerpa, whose whole form 

 and existence is determined by the cell- wall ? 



Secondary and unessential the cell-wall certainly is, however, 

 when we compare it with the protoplasm and nucleus. 



The latter determine whether a structure is living or dead ; the 

 former merely influences the manner of life. 



Caulerpa as a genus is undoubtedly dependent on the presence 

 of a membrane, but Caulerpa as a living thing is due to the co- 



1 " Zur Geschichte and Kritik <les Zcllcnbegritfes in der Botanik," yon Dr Adolph 

 Hansen. Giessen: .1. Kicker, 1897. 



