CHAPTER II 



PHYSIOLOGICAL MORPHOLOGY. 



SECTION 6. The Structure and Function of Plant-organs. 



WITH the exception of certain low organisms, all plants undergo 

 more or less marked morphological differentiation. It is a very suggestive 

 fact that this morphological differentiation develops in close correlation 

 with division of labour, that is to say, with the modification of particular 

 parts to perform particular functions. Thus, in order that a root may 

 properly perform its special duties, it must be differently constituted 

 to a shoot, while a leafy branch must be different in form to one bearing 

 flowers. It does not, however, come within the scope of a textbook of 

 general Physiology to describe the details of the relationship between 

 form and function, and moreover a knowledge of vegetable anatomy and 

 morphology is assumed on the part of the reader 1 . 



A differentiation into root and shoot can almost always be recognized. 

 This differentiation is directly due to the fact that nearly all plants pass 

 through their entire life-history permanently fixed to one spot. The 

 organs which absorb inorganic or organic nutriment from the substratum 

 on which the plant grows, and at the same time form an anchoring attach- 

 ment, have naturally a very different form and structure from the subaerial 

 organs which subserve entirely different functions. Such differentiation 

 may be found even in non-septate plants, as Botrydium and Mucor ; 

 though, owing to their simple structure, the differentiation into distinct 

 members is clearly not as marked as it is in higher forms. In these latter the 

 subdivision of the shoot into leaves and branches, together with the large 

 amount of surface which the former present, ensures the maintenance 

 of favourable conditions for the functional activity of the chlorophyll 

 mechanism. These conditions are a sufficiently strong illumination, and 

 an adequate supply of carbonic acid gas. Since, however, the amount of 

 transpiration is increased by the same arrangement, it is easy to understand 



1 In addition to the usual textbooks attention may be called to Goebel, Vergleichende Ent- 

 \vickelungsgeschichte d. Pflanzenorgane, 1883; Sachs, Vorlesungen u'ber Pflanzenphysiologie, 1887, 

 2. Aufl., p. 3 [Eng. Trans, by H. Marshall \Vard, Clarendon Press, 1887, p. 3 seq.]. 



