2 GROWTH 



Hence the terms Photomorphosis, Chemomorphosis, Barymorphosis, merely 

 indicate the forms of growth resulting from special external stimuli, 

 and convey no information as to the internal factors involved, or as to 

 the precise manner in which the changed mode of growth is produced. 

 Nevertheless the various growth-reactions to special stimuli indicate the 

 existence of specific powers and activities, and hence may enable the 

 conditions for the existence of a particular organism to be determined, 

 or may enable us to decide the importance of special external conditions 

 in accelerating or retarding growth, or in modifying its character. 



After a permanent form has been reached, and external growth has 

 ceased, internal growth may still be possible. Instances of this are 

 afforded by the sliding growth of certain cells, by the thickening of cell- 

 walls, and by the intracellular formation and growth of starch-grains, as 

 well as of crystals of proteids and of calcium oxalate. In the last two 

 cases we are probably dealing with a simple process of crystallization, 

 requiring only the accumulation or continued production of the crystallizing 

 substance. On the other hand, the formation and growth of the cell-wall, 

 and possibly also of starch-grains, are the result of protoplasmic activity. 

 The production of a cell-wall composed of non-living substance is not 

 however an essential condition of life, and it is precisely in the case of 

 partial functions of this kind that causal relationships can most easily 

 be traced. The complex mechanism of the protoplast, its mode of growth, 

 and that of its organs form the fundamental and also the most difficult 

 problems connected with vital activity. The varied mutual influences 

 of cells upon each other depend upon the specific nature of the protoplasts 

 they contain, and these influences may be such as to produce temporary 

 or permanent modification in the structure of the protoplasts in question. 

 Hence it is clear that we can more easily determine the means by which 

 different cells and different organs influence each other, than we can the 

 related processes taking place within the protoplast. 



The different forms of growth may be classified according to their mode of 

 origin, according to their biological importance, and according to the character 

 of the external changes. Special terms are however not necessary for growth 

 with and without cell-division, or for growth involving cell-fusion \ 



Growth by intussusception and by apposition are not always clearly distinct, 

 and may also be accompanied by sliding growth. 



The term growth may be used to indicate all formative processes leading 

 to a change of shape or of structure. Growth is usually accompanied by an 

 increase of volume, even although this may be due solely to the enlargement 



1 Cf. Wiesner, Elementarstructur, 1892, pp. 194, 222; Roux, Ergebnisse d. Anatomic u. Ent- 

 wickelungsgesch. von Merkel u. Bonnet, 1892, Bd. n, p. 434 ; Programm d. Forschnngsmethoden 

 d. Entwickelungsmechanik, 1897, p. 28. The occurrence of different modes of growth was mentioned 

 by Meyer, Pflanzenphysiol., 1838, Bd. n, p. 336. 



