602 



SCIENCE 



[N. S. Vol. XLIX. No. 1278 



tragacantli would be less soluble, and a group 

 like agar, for example, would not appear to be 

 soluble at all. It is by no means implied that 

 solubility invariably depends upon diiJerences 

 in the carbohydrate component as it might 

 also result from the character of the amino- 

 compounds or proteins present, especially in a 

 protoplasm rich in nitrogen. 



My studies of growth have been carried out 

 on the assumption that the principal features 

 of importance are those which might be due to 

 the reactions of the carbohydrates and of the 

 proteins which may be present. It is in order 

 therefore to inquire into the condition in 

 which these substances may occur in living 

 matter particularly with respect to their rela- 

 tion to each other. The first and most im- 

 portant relation to be considered is the fact 

 that the mucilages or pentosans and the al- 

 bumins of amino-compounds of the cell may 

 diffuse into each other very slowly or not at 

 all. Their joint presence in living matter is 

 in a condition in which they are intimately 

 mixed in a colloidal condition. Molecules or 

 groups of molecules of each lie side by side 

 with various possible arrangements. Thus it 

 is conceivable that the mucilage of a cell 

 might be in the form of a mesh or honeycomb 

 with the proteins forming droplets enclosed in 

 the continuous structure, or the reverse might 

 be the case; again substances of both groups 

 might each form a continuous meshwork inter- 

 locking with the other, and another category 

 of variables would be introduced by the lipins 

 which might be interposed or incorporated in 

 these systems. Living matter probably does 

 not remain fixed in any one of these simple 

 arrangements, or in any one of a dozen others 

 which might be described if space permitted, 

 and the suggestion is ventured that the play 

 of molecular force where aggregates of a 

 different kind are in contact may constitute 

 the essential and characteristic action of 

 living matter. 



Let us now fasten attention upon the the- 

 oretical final structure of protoplasm and en- 

 deavor to construct for ourselves a mode or 

 plan of action which might be followed in 

 its growth. Growth as has been defined con- 



sists of two processes. First the molecules or 

 aggregates of molecules of the two kinds, the 

 carbohydrates and the albumins, combine with 

 and absorb water, thus increasing the volume 

 of these units regardless of whether such mole- 

 cules be in the form of droplets or fibrillse of 

 a meshwork. Instances of growth are known 

 in which water only has been added to the 

 colloidal structure in which in all probability 

 the solid particles have been variously re- 

 arranged. In general however growth is ac- 

 companied by the accretion of molecules of 

 solid material in such manner that as develop- 

 ment proceeds their proportion to that of the 

 water taken increases and organs are then 

 said to show an increase of relative dry weight 

 with age. 



On the other hand, my own studies have 

 shown that succulent organs or stems, such as 

 leaves of the Crassulacese, joints of cacti, 

 fruits, etc., do not show such increase and the 

 proportion of solid matter and of water 

 undergo but little change, their incorporation 

 being at a rate which keeps them near the 

 initial proportion. It is suggested that such 

 action may be shown by the fleshy fungi al- 

 though I have not seen any data bearing di- 

 rectly upon this matter. 



The conditions under which hydration may 

 ensue are by no means identical for the two 

 main constituents of living matter. Thus the 

 albumins and their derivatives as exemplified 

 by the behavior of gelatine show a swelling 

 determined or facilitated by the hydrogen ion 

 concentration or acidity of the solutions, be- 

 ing increased as this rises. The pentosans, on 

 the other hand, show no such increase, and 

 being weak acids, their hydration is retarded 

 by the hydrogen ion. The swelling of a mix- 

 ture of the two will therefore be a resultant of 

 these effects and of the proportion of the two 

 elements in the living mixture, and as the un- 

 ceasing action of respiratory meta'bolism re- 

 sults in the formation of some residues of 

 acids, the condition of hydration of any mass 

 of protoplasm may be said to reach a volume 

 determined by these opposed reactions. The 

 effects in question may be illustrated by the 

 citation of my experiments, in which gelatine 



