52 THE CELL 



In the case of every tissue, namely, the size of the cells, no less 

 than their form and structure, must be adapted to their physiological 

 activity. Every cell accordingly exhibits specialisation for a definite 

 purpose, not only with regard to its morphological features, but also 

 in respect of its dimensions. 



It is readily comprehensible, for example, why storage cells are, in 

 general, much larger than photosynthetic elements. Practically the 

 entire lumen of a cell can be utilised for the deposition of reserve- 

 materials ; chloroplasts, on the contrary, are necessarily restricted 

 to the immediate vicinity of the cell- wall, so that in photosynthetic 

 cells the available cell- wall surface should be as large as possible. 

 Consequently, the cell-walls are multiplied in photosynthetic tissues 

 and as a result the size of the individual cells is reduced as 

 far as is compatible with their general vital activity on the one hand, 

 and with a proper degree of illumination on the other. The pro- 

 nounced elongation, which is in general characteristic of bast-fibres, 

 must also be regarded as a factor in the specialisation for mechanical 

 purposes exhibited by these cells, since elongation of the individual 

 elements of a fibrous strand undoubtedly increases the strength of the 

 whole bundle. Differences in the width of water-conducting vessels 

 are similarly correlated with the varying requirements of the plant in 

 respect of the amount and velocity of the transpiration current. 

 Wind-borne pollen-grains, again, are generally smaller than those which 

 are adapted for transportation by insect agency. In all the preceding 

 instances, and in many others of a like nature, the adaptive character 

 of the prevailing cell-dimensions can be readily demonstrated. 



D. ORIGIN AND ADVANTAGE OF CELLULAR STRUCTURE. 



There are quite a number of Thallophyta, comprising both Algae 

 and Fungi, which do not possess the typical cellular structure that 

 alone has been under discussion so far ; in these cases the plant-body 

 consists of a single undivided protoplast enclosed within a continuous 

 membrane. This unsegmented condition of the protoplast is charac- 

 teristic of many Schizophyta and Protococcales and of all the 

 Desmidiaceae, Diatomaceae, Siphoneae and Phycomycetes. Such 

 plants are often termed unicellular in contrast to the multicellular 

 forms which exhibit the typical cellular structure. This terminology 

 implies that all plants without exception can be included in the 

 " cellular " scheme. The term unicellular, however, is justifiable only 

 when the plant to which it is applied is really homologous with a 

 single cell of the most nearly related multicellular species, or where in 

 other words the multicellular condition owes its phylogenetic origin 

 to the association of a number of unicellular individuals. Ontogeneti- 



