ADVANTAGES OF CELLULAR STRUCTURE 55 



many struts, which prevent the collapse of the delicate tube of cellulose : 

 incidentally, they divide the tube into a series of compartments and 

 the protoplast into an equal number of segments. The simple tubular 

 plant-body has, in fact, become transformed into a cellular filament. 



A second factor which determines the cellular structure of plants, 

 and, indeed, of organisms in general, is the fundamental physiological 

 principle of division of labour. From the mechanical standpoint the 

 interpolation of solid walls is the main object of septation, the sub- 

 division of the protoplast being merely an inevitable consequence of 

 the septate condition. From the point of view of division of labour, 

 on the contrary, subdivision of the continuous protoplast is the primary 

 purpose of cellular structure, while the formation of cellulose septa 

 merely represents the means employed to gain this end. The 

 partition of a large protoplast gives rise to a number of separate 

 structural units, each capable of independent physiological activity ; 

 such an association of functional units is far better adapted than a 

 single large unsegmented protoplast for dealing effectively with a 

 number of diverse functions. 



The beginnings of this devolutionary process of septation are 

 descernible even among the Siphoneae and Phycomycetes. The plant- 

 body of Vaucheria may, as far as its vegetative condition is concerned, 

 be described as a variously-branched, non -septate tube. In connection 

 with the reproductive processes, however, the living substance which is 

 destined to give rise to asexual or sexual germs, usually becomes 

 segregated from the somatic protoplasm. In Vaucheria sessilis, 

 V. tubcrosa, V. gemmata, etc., asexual propagation is initiated by a 

 swelling up of the distal ends of certain branches ; the protoplasmic 

 contents of each swelling become completely separated from the rest 

 of the protoplasm by a transverse wall before giving rise to a zoospore. 

 The male and female sexual organs (antheridia and oogonia) likewise 

 become shut off from the vegetative region of the thallus. Among the 

 Phycomycetes, also, the formation of sexual or asexual reproductive 

 organs is always initiated by the appearance of a number of irregularly 

 disposed transverse walls, which divide the previously continuous 

 mycelial cavity into several compartments. In such cases the formation 

 of septa cannot be explained on mechanical grounds ; for in neither of 

 the instances described does the awakening of reproductive activity 

 necessitate any addition to the mechanical strength of that part of the 

 plant-body in which septa appear. On the contrary, the septation 

 here undoubtedly represents a secondary consequence of a physiological 

 differentiation of the protoplasm. The dissociation of the reproductive 

 and vegetative functions of the living substance, and the accompanying 

 separation, by means of cell-walls, of the organs or protoplasmic 



