2 METABOLISM 



the expenditure of a calculable amount of mechanical energy. Moreover, we 

 shall discover that organisms expend energies other than mechanical, such 

 for example as heat, light, and electricity. These energies must have entered 

 the organism in some form or another and must have undergone transforma- 

 tion within it. It follows that, in the organism, we meet with Transformation 

 of energy. 



3. The third series of changes are those which are so familiar in the animal 

 world, viz. the absorption of materials from the external world, their trans- 

 formation within the organism and the excretion of certain of these transformed 

 bodies, that is to say, the changes summed up in the term Metabolism. In 

 plants, also, a similar metabohsm is observable, although special expedients 

 are sometimes requisite to demonstrate its occurrence. 



In the following pages we have to treat of transformation of form, of 

 energy, and of materials, and we may most conveniently commence our studies 

 by considering the last of these, transformation of materials or metabolism. 

 A discussion of the physiology of organisms in general is outside our present 

 task ; we will confine our attention to the physiology of plants only. At the 

 same time it must be borne in mind that the distinctions once believed to 

 exist between the physiology of plants and the physiology of animals have 

 become more and more obliterated, and that it has already become possible to 

 elaborate a general physiology ([Bernard, Lemons sur les phenomenes de la vie 

 communes aux animaux et vegetaux. Paris, 1878-9] Verworn, 1894). 



Before we enter on the discussion of the first great division of our subject, 

 a few words on the methods of plant physiology will not be out of place. These 

 methods are identical with, or at least do not differ in any essential particular 

 from, those of physics and chemistry. In the first place, observation, and that 

 too of the most exact character, is necessary for the proper study of the changes 

 occurring in the organism ; but observation alone is insufficient for the accu- 

 rate determination of the causes of these changes. Plant hfe, as we shall 

 discover, is maintained only in the presence of a whole complex of conditions, 

 and it is only rarely possible to carry out a physiological observation under 

 conditions of such a nature that we can say with confidence that a certain 

 change takes place in the plant when, and only when, accompanied by a single 

 change in the environment ; then only can it be said that the special alteration 

 in the surroundings is the cause of the special phenomenon in the plant. We 

 have to employ the utmost care in contriving that only one of the many factors 

 which affect the plant is altered. Observations made under such conditions 

 are termed experiments. Owing to the nature of the case, physiological experi- 

 ments are generally restricted within narrower limits than those of physics and 

 chemistry. A purely physical experiment in plant physiology has for that 

 very reason not infrequently led to most serious error, as an example will show. 

 If a physicist were to fasten a wire by one end to some fixed point in a vertical 

 position, and attach a metalhc knob to the other, he might reasonably 

 deduce that the wire bent over in consequence of the weight of the knob, and 

 his deduction would be confirmed if, on removal of the knob, the wire once 

 more became straight. A similar bending is seen in the peduncle of the flower- 

 bud of the poppy, and we might very well conclude that curvature there 

 also was due to gravity acting on the bud. If the bud be cut off, as in the 

 case of the knob in the physical experiment above described, the peduncle, 

 it is true, becomes straight, so we naturally conclude that the bud actually 

 pulls the passive peduncle downwards. Vochting (1882), however, has 

 shown that when the weight of the bud is upheld the peduncle bends all the 

 same, and that this curvature is maintained even when the pull from above 

 more than compensates the weight of the bud. A consideration of these facts 

 leads us to the conclusion that the weight of the bud has nothing to do with 



