GROWTH UNDER CONSTANT EXTERNAL CONDITIONS 3 



of the intercellular spaces, and involve no increase of organic substance. A 

 permanent change of shape may however occur without any change of volume, 

 as for example when the growth in length of an organ is just counterbalanced 

 by its decrease in diameter, 



In all cases growth is a complex form of vital activity, in which metabolism, 

 transformations of energy, and the incipient growth-changes themselves mutually 

 influence and regulate each other. A supply of energy and of constructive material 

 is therefore essential to growth, but even when both are available, growth may 

 temporarily or permanently cease unless the protoplast can utilize them in the 

 proper manner. The fact that a continuous supply of plastic material forms an 

 essential condition for progressive growth, at once shows the importance of 

 adequate nutrition. 



Growth and development form an integral part of Physiology, and the study 

 of the causes underlying them involves that of vital activity in general. The 

 matter has long been treated from this point of view by botanists \ and as much as 

 a century ago numerous researches were performed which had as their aim the 

 causal explanation of particular processes of growth. 



Animal physiologists have hitherto mainly confined themselves to the study 

 of the adult animal, although Bernard 2 pointed out that a causal explanation 

 of the facts of development formed one of the tasks of Animal Physiology. 

 Recently however numerous experiments have been made in this direction by 

 zoologists 3 . There is however no reason for considering 'the mechanics of 

 development,' as Roux calls it, to form a subject apart from Physiology. 



The same problems and methods of research apply to animals as to plants, 

 and many of the theoretical considerations of Driesch, Roux, and others deal with 

 factors which have not only been long recognized in the case of plants, but have 

 even found practical use. 



SECTION 2. Growth under Constant External Conditions. 



The entire character of the development of a plant and of its individual 

 organs suffices to show that all the peculiarities of external form and internal 

 structure are the result of the self-regulation of growth as regards its amount 

 and character, as well as its localization in time and space 4 . In most plants 



1 Cf. the first edition, 1881, Bd. I, Introduction; Bd. II, chaps, iv, v; Sachs, Lehrbuch der 

 Botanik, 1873; Vb'chting, Ueber Organbildung im Pflanzenreich, 1878, pp. 4, 241. In these works 

 not only are the attained facts discussed, but also the future aims of research are indicated. Hof- 

 meister (Allgem. Morphol., 1868), in spite of numerous errors in detail, made a praiseworthy attempt 

 to place the facts of morphology and development upon a physiological basis. On the other hand, 

 Nageli (Die Starkekorner, 1858) attempted to derive the shape and growth of starch-grains from the 

 molecular reactions which give rise to them, and thus to explain their growth upon a molecular basis. 



2 Bernard, Le9ons sur les phe'nomenes de la vie, 1885, Vol. I, p. 390. 



8 Roux, Programm u. Forschungsmethoden d. Entwickelnngsmechanik, 1897, p. 171, and the 

 literature there quoted. On the scope of Physiology, cf. Vol. i, p. 8 of this book. Also Driesch, 

 Ergebnisse d. Anat. u. Entwickelungsgesch. von Merkelu. Bonnet, 1898, Bd. viil, p. 712. 



* We are not concerned either with the details of adult structure, or of development. Cf. 

 Hofmeister, Allgem. Morphologic, 1868 ; Goebel, Vergl. Entwickelungsgeschichte d. Pflanzenorgane, 

 1883 ; Organography, 1900. 



B 2 



