10 INTRODUCTION 



Again, quantitative experiments carried out in order to test the 

 relative impermeability of cork towards water and gases throw 

 additional light upon the protective function which lias been attributed 

 to corky tissues on the strength of their histological peculiarities. 

 In this way, the results of anatomical investigation are both amplified 

 and rendered more exact by the proper application of the experimental 

 method. In fact, the utility of experiment from the point of view 

 nf the physiological anatomist largely depends upon the extent to 

 which this met Ik id of research enables him to replace more or less 

 speculative generalisations by well established " laws." No one, 

 accordingly, who works in the field of physiological anatomy, can 

 afford to dispense with experimental methods; at the same time, it 

 will not do to overestimate the capabilities of this instrument of 

 research. As a matter of fact, the experimental method is, in this 

 branch of biology, subjected to two important restrictions. 



In the first place, there are many questions concerning the relation 

 between structure and function which do not admit of experimental 

 treatment at all. How, for example, coidd the experimental method 

 be applied with a view to determining the significance of such minute 

 structures as the fianges of arm-palisade cells and the cuticular ridges 

 of guard-cells, or of features like the parenchymatous shape of fibrous 

 elements, the thickening of the edges of collenchymatous cells, the thin- 

 walled character of tactile papillae, and many similar arrangements ? 

 The microscopic size of an object does not of course in itself place 

 an insuperable obstacle in the w T ay of experimental investigation. In 

 the case of the stinging hairs of the Nettle, for example, it can 

 readily be shown by experiment that the thin strip of the wall 

 situated immediately below the head of the hair actually repre- 

 sents the line of rupture. Again, the protoplast of an Algal cell 

 of microscopic dimensions can be divided by plasmolysis into two 

 portions, of which only one contains a nucleus; in this way, the 

 difference of behaviour between nucleated and non-nucleated cyto- 

 plasm can be observed. Chloroplasts even may be isolated and 

 subjected to various tests in order to show that they do not lose 

 their photosynthetic capacity in such circumstances. There are, 

 however, just as many instances in which the physiological importance 

 of microscopic features cannot be investigated by experimental means ; 

 where this is the case, it is permissible to base definite conclusions 

 upon the data of comparative anatomy alone. 



A further limitation of the experimental method and one which 

 is often not sufficiently appreciated arises out of certain fundamental 

 properties of the living organism. It is characteristic of physiological 

 experiments that they expose an organism, or a portion thereof, to 



