54 THE CELL 



the nature and extent of these advantages is the same, whether the 

 many-celled condition of a large and highly organised plant owes its 

 origin to the (phylogenetic) aggregation of one-celled individuals or 

 germ cells, or whether it is on the contrary due to the secondary sub- 

 division of a non-cellular plant-body into a number of protoplasts 

 enclosed in separate compartments. 



It is mainly in connection with the mechanical requirements of the 

 plant-body that cellular structure is highly advantageous. If a plant 

 is to maintain a constant shape, and if it is to provide itself with 

 permanent members, it must develop a comparatively solid skeletal 

 framework, within which the shapeless living substance can arrange 

 itself in a definite and orderly manner. In the absence of such a 

 framework or skeleton of solid material, it would be impossible for a 

 large plant to attain to any considerable degree of external organisation. 

 The truth of this assertion is very clearly illustrated by the familiar 

 case of the Myxomycete-plasmodium ; in the living condition, the Plas- 

 modium, which is, of course, merely a naked, unsupported protoplast of 

 unusual size, is continually changing its outline, or, in other words, 

 represents a protean structure devoid of permanent shape. Even among 

 plants of microscopic size, such as the Schizophyta and Proto- 

 cocgales, the maintenance of a definite shape would be impossible if 

 the protoplast were not invested by a cellulose membrane. The evolu- 

 tion of such non-cellular forms as the Siphoneae and Phycomycetes 

 may be figuratively described as an experiment on the part of Nature, 

 which was intended to test how far the mere possession of a cellulose 

 " exoskeleton " would enable a large plant to achieve a certain degree 

 of external differentiation. As a matter of fact even many Siphoneae 

 find this simple type of skeleton insufficient for their needs. In the 

 genus Caulerpa, for instance, to which reference has already been made 

 more than once, numerous trabeculae of cellulose are developed in all 

 parts of the plant-body ; these structures constitute an efficient but- 

 tressing system, which, as Janse has shown, 25 seems particularly designed 

 to preserve the plant from deformation by the force of its own tur- 

 gescence. Arrangements of this kind would, however, be of little value 

 in the case of terrestrial plants, which require the much more effectual 

 support provided by complete partitions ; as it is further desirable that 

 the walls should be disposed in several intersecting planes, the plant- 

 body has become divided into numerous compartments, or, in other 

 words, has acquired a cellular structure. It is by no means necessary 

 to turn to the land-plants, with their exacting mechanical requirements, 

 in order to illustrate the principle under discussion. Any filamentous 

 Alga will afford a simple instance of its application. Here the 

 transverse walls, that are developed at regular intervals, represent so 



