58 PLANT LIFE 



infinitely beyond that to be encountered 

 in any alga. Furthermore, the organs of 

 attachment in the land plant no longer serve 

 merely as " holdfasts," but they discharge 

 important functions in connection with the 

 absorption of water and mineral food-supplies. 

 Their structure becomes increasingly modified 

 with reference to the larger functions they 

 have to discharge. 



In another respect, also, the higher plants 

 differ from the lower, namely in the greater 

 degree of definiteness with which their 

 various organs are produced. In other words, 

 the organisation of the individual is as a whole 

 more specialised, and is less apt than are 

 simpler types to vary its normal sequences 

 of growth. The different morphological 

 structures are less and less susceptible of 

 alteration than is the case with more primitive 

 plants, in which the bonds of correlation and 

 co-ordination between the constituent cells 

 and tissues are weaker. 



If we ask why there should be this advanced 

 degree of anatomical differentiation associated 

 with a land habitat, we shall find the answer 

 to lie on the one hand chiefly in the needs 

 for adequate water supply and all that this 

 involves, and on the other in the demand for 

 a body constructed on sound mechanical 

 principles, so that it may be enabled success- 

 fully to withstand the various stresses and 

 strains to which it is continually liable to be 

 subjected. 



