76 Morphology BOOK i 



forms of the simpler plants. The cell increased in size ; 

 at a certain stage the ratio between bulk and surface was 

 found insufficient, and the cell divided. If the two resulting 

 cells failed to separate, the cell-filament was initiated ; 

 later, for similar reasons, the cell-plate was formed, and 

 later still the cell-mass. Increase of size of the latter 

 could not now be adjusted by separation into two, and 

 hence the operation of branching came into being. Internal 

 differentiation, which was unnecessary so long as the body 

 remained small, became imperative as size increased. 

 Among the Algae of the present day this process can be 

 traced, and the beginning of such differentiation is visible 

 in such plants as Fucus, where we have a protective outer 

 layer and a rudimentary conducting-system in the centre. 

 Though much branched, the general anatomy is alike in 

 both axis and appendages. In the larger forms the differen- 

 tiation of the axis has become more conspicuous, but all 

 stages can be traced. The determining need seems to 

 have been to maintain an adequate ratio between surface 

 and bulk, a matter of increasing difficulty as the dimensions 

 of the plant increased. The form attained has therefore 

 depended on the division of labour which the larger size 

 has involved, so that all parts of the plant may partake 

 of the advantages offered by the environment. When 

 migration to the land took place physiological conditions 

 determined the form assumed. The plant, originally a 

 flat thallus, gradually assumed permanently what had 

 before been only occasionally attained the upright position, 

 and the subdivision of its form increased as the environment 

 became air instead of water. The so-called leaves of the 

 Liverworts and Mosses, though little more than emphasized 

 subdivisions of the original flat thallus, began to show 

 a rudimentary differentiation, while the initiation of 

 a root system is indicated by the differentiated rhizoids 

 which mark the bases of the higher mosses. The physio- 



