iv] THE SECOND GRADE 89 



be contained in a small fraction of a cubic millimetre 

 of substance, and this way has led up a blind alley. 

 One tribe of plants, the Siphoneae, has made a brave 

 attempt to gain size while still remaining a single 

 cell 1 . A plant of Caulerpa, for instance (one of these 

 sea-weeds), may have several square feet of surface, 

 and in spite of being one continuous piece of proto- 

 plasm with a wall of cellulose round it, is differentiated 

 into organs resembling in appearance and no doubt 

 in function the stem, leaves, and roots of higher plants. 

 Needless to say, it has only been able to attain this 

 relatively huge bulk by restricting itself to growth in 

 two dimensions only, and is quite thin and plate-like 

 throughout. 



What possibilities of development lay along this 

 line we cannot say ; all we know is that actually it 

 has not led far. The real advance has been made 

 in a quite different way ; by keeping the cell's 

 original form and plan, but joining up a number of 



1 I am aware that botanists distinguish between cells, which have 

 one nucleus, and coenocytes, or masses of protoplasm with many 

 nuclei, such as are found in Caulerpa and other Siphoneae. However, 

 I am using the word cell in a wide sense, a sense dictated by the 

 historical or evolutionary point of view, to denote a discrete mass of 

 protoplasm isolated by natural causes, and if this definition be 

 allowed, then Caulerpa is simply a single cell which has found out 

 the way to become large. The number of nuclei in a cell is often 

 quite unimportant: in the Protozoa one form may have a single 

 nucleus, while a close relation has several. 



