THE COMPARATIVE AND EXPERIMENTAL METHODS 9 



In any case, however, it is impossible to deny the actual existence 

 of purposeful adaptations. Physiological anatomy describes the adap- 

 tive features of the internal structure of plants, but is incapable of 

 explaining their origin, and indeed does not profess to do so. 



A few words must next be devoted to the general methods of 

 investigation employed by the physiological anatomist. In many 

 cases, an application of the methods of comparative anatomy suffices 

 to disclose the physiological significance of a given tissue or arrange- 

 ment, and to indicate the connection between its structure and its 

 function. In the case of cork, for example, mere observation of the 

 histological features of the cells and of their characteristic disposition 

 at once leads to the conclusion that one is dealing with a protective 

 dermal tissue. Similarly, the manner in which the photosynthetic 

 cells of a leaf are linked up with the parenchymatous bundle-sheaths 

 immediately suggests that the latter serve as paths for the transloca- 

 tion of synthetic products. Finally, the fact that the ventilating 

 spaces of a leaf communicate with the outer atmosphere by means 

 of the stomata can be determined by mere inspection. 



Frequently, it is impossible to determine the principal function of 

 a tissue with certainty from anatomical evidence alone. In these 

 circumstances resort must be had to experiment. The thick-walled 

 character of fibrous cells, for example, is not in itself sufficient proof 

 that the elements in question are specialised for mechanical purposes ; 

 but it is permissible to ascribe such a function to fibrous tissue 

 when its great tensile strength has been demonstrated by experi- 

 ment. Similarly, the existence of a photosynthetic tissue-system was 

 not recognised until long after the chloroplasts had been shown to be 

 organs of photosynthesis by means of physiological experiments. 

 Again, the view held by the author, according to which the aleurone- 

 layer of grasses is a glandular tissue engaged in the secretion of 

 diastase, is largely founded upon experimental evidence. 



The experimental method is also invaluable to the physiological 

 anatomist as a means of confirming, of correcting, or of supplementing 

 the results obtained by the methods of comparative anatomy. One 

 or two examples will add force to this argument. The demonstration, 

 by means of chemical tests and physiological experiments, of the fact 

 that synthetic products travel in the parenchymatous bundle-sheaths, 

 provides an experimental confirmation of conclusions previously arrived 

 at on anatomical grounds. Similarly, microscopic investigation of one 

 of the sensitive bristles of Dionaea muscipula (Ch. XII.) suggests that 

 the structure in question is likely to suffer the greatest amount of 

 deformation, when it is bent, in the constricted " hinge-area " ; an 

 actual trial at once demonstrates the correctness of this inference. 



