HORMONES, DRUGS, AND TOXINS 723 



Hormones in Plants. Although there is no such effective way of chemical 

 interchange in plants as there is in the circulating blood of animals, there is 

 distinct evidence that chemical products of one part are able to influence the 

 activities of other parts. 



The lateral roots, which normally grow horizontally, can be made to grow 

 vertically downwards if the main root is removed. Errera (190t) investigated, in 

 pines, the corresponding change of direction of growth of a branch into a vertical 

 stem when the apical bud of the main stem is removed. He suggested that the 

 apical bud of the main stem forms some kind of an internal secretion, which 

 prevents the upward growth of the lateral shoots as long as this apical bud is 

 present. 



Keeble (1910, pp. 135-137) considers that such "chemical stimulators" play a 

 part in the transfer of the activity of localised cambium cells to others in their 

 neighbourhood. In the case of Convoluta Roscoffensis, the signal for the com- 

 mencement of the later phases of development owes its origin to the presence of 

 the green algal cells, without whose concurrence, probably by the production of a 

 hormone, no kind of artificial feeding has been found to be effective. 



Mention may also be made of the substance extracted by rain from grass, 

 which has been shown by Pickering (1911, see also Russell, 1912, p. 112) to be 

 injurious to apple trees. They should not, in fact, be surrounded by growing grass, 

 as is common in orchards. 



THE ACTION OP DRUGS AND OP SOME OTHER CHEMICAL COMPOUNDS 



There are some of these substances which will receive mention in the present 

 section for two reasons. The first is that, although the subject properly belongs 

 to pharmacology, it is clear that the mode of action of the hormones of the previous 

 section cannot be understood until we know more of the action of drugs on cells. 

 The second is that certain alkaloids and ot&er active principles are of great value 

 as means of investigation, owing to their action as excitants or paralysers of 

 particular kinds of cells or nerve endings. 



Their Mode of Action. The preparation by Barger and Dale (1910) of a 

 series of amines, which were found to possess the power which adrenaline has of 

 stimulating sympathetic endings, but in different degrees, gave the opportunity of 

 comparing this property with their chemical structure. Details of the latter will 

 be found in Barger's monograph (1914). It was found that approximation in 

 structure to that of adrenaline was associated with increased intensity of action, 

 and more definite restriction to the sympathetic system. The optimum carbon- 

 skeleton for this purpose consists of a benzene ring with a side-chain of two carbon 

 atoms, of which the terminal one is attached to an NH 2 group. This is further 

 intensified by the presence of two hydroxyls on the benzene ring in the 3 : 4 position 

 relative to the side-chain. These substances are, therefore, catechol derivatives. 

 Of these bases, those with a methyl-ammo group, including adrenaline, produce 

 the inhibitory effects of the sympathetic, such as that on the intestine, more 

 powerfully than the excitatory effects on the blood vessels, etc. The opposite is 

 true of the primary amines of the same series. It is to be noted, however, that a 

 catechol nucleus is not essential. Catechol itself has no action of the kind 

 referred to ; while not only parahydroxyphenyl-ethylamine, but also iso-amylamine, 

 are powerfully active. 



As has often been pointed out, in comparing the activity of a series of related 

 substances it must not be forgotten that, in altering the chemical composition, 

 we alter in many ways the physical properties also. We have to reckon with 

 changes in solubility, in ability to pass through the cell membrane, in approxima- 

 tion to the colloidal state, in surface tension, in rate of diffusion, and so on. 



Barger and Dale, in the paper mentioned, give a valuable discussion of the 

 theoretic aspect of the question, from which I take the following considerations. 

 They show that the ease of oxidation has nothing to do with activity. Since 

 there is evidence that the excitatory and inhibitory effects can be varied 

 independently, reason is given for the belief that the myo-neural junctions 



