38 PHYSIOLOGICAL MORPHOLOGY 



that the plant may sacrifice a portion of its power of CO 2 -assimilation in 

 order to avoid a dangerous loss of water. This is attained by reducing 

 the amount of surface exposed, and may or may not be accompanied by 

 a disappearance of the leaves and an increased development of cuticle, 

 while, at the same time, chlorophyll usually develops in the peripheral 

 and most favourably situated tissues. Such compromises are everywhere 

 necessary in order that the different organs may work harmoniously 

 together, and that the conditions necessary to the life of the organism may 

 be created and maintained. 



As is well known, the leaves are much reduced in non-green plants, and 

 this affords distinct evidence that the increased surface presented by ordinary 

 leaves is of value primarily to the chlorophyll mechanism. At the same 

 time, the frequently remarkable development of the perianth segments is 

 in no wise inconsistent with this conclusion, for the perianth segments 

 are leaves specially modified for quite different purposes and functions. 

 Organs of similar origin frequently diverge markedly from one another in 

 habit, form, and function. Thus rhizomes, tubers, corms, &c. are all 

 modified stems which have adopted a subterranean mode of life for 

 protection against prolonged cold or drought ; and similarly foliar organs 

 are made to serve widely different functions in all higher plants. It is 

 sufficient to mention carpels, stamens, floral leaves, prophylls, tendrils, 

 thorns, &c., to realize how manifold are the functional and structural 

 modifications which a foliar organ may undergo. 



Owing to the fact that we select as types the most highly differentiated 

 forms, we may speak of plants as being rudimentary in comparison with these, 

 if they have not yet reached this high degree of development, and, again, of 

 reduced forms which have become simple by degeneration from a higher 

 condition. It is in full accordance with the theory of descent, that an early 

 diverging line of development, such as terminates, for example, in the 

 common toadstool, should at no period show any marked parallelism to 

 the line which terminates in a flowering plant. In all cases the need 

 of the plant to fix itself and absorb nutriment must necessarily lead 

 to the formation of anchoring and absorbing organs, which, it may be 

 noticed, are present even in those fungi which pass their entire existence 

 underground and in darkness. An externally visible segmentation is by 

 no means an essential requirement, as is shown by the existence of simple 

 algae and fungi, which are spherical and unicellular even when adult. Such 

 non-rooted organisms frequently develop cilia or flagellae, as locomotory 

 organs by means of which movements of translocation may take place. 

 Morphological differentiation into vegetative and reproductive organs is 

 also possible in plants which are not fixed to a particular substratum. 

 Since, however, reproductive organs require air, both for the distribution of 

 the spores, &c., and in order that full development may take place,, we find 



