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. 
