394 NATURAL SCIENCE - eseeien 
fluence 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. 
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. 60-58, 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 hving thing is due to the co- 
*‘Zur Geschichte und Kritik des Zellenbegriffes in der Botanik, ’ * yon Dr Adolph 
Hanson Giessen: J. Ricker, 1897. 
