PREFACE. 



Three main conceptions concerning growth and its developmental 

 aspects in plants are to be met in the history of physiology in the half 

 century beginning with the researches of Sachs and his school. The 

 first or earliest, that of special stuffs or substances necessary for the 

 initiation of growth and differentiation of various organs, especially 

 for the origination and development of reproductive organs, is now 

 giving way to the modern conclusion that "formative" material as 

 such has no actual existence in fact and no good basis in theory. The 

 present trend of thought leads to the assumption that growth 

 proceeds from and depends upon states or combinations of material 

 or accumulations in connection with living matter rather than upon 

 any special constructive stuff or substance. This may also be held 

 to apply to hormones, vitamines, and other symbolic expressions for 

 combinations of material necessary for initiating and maintaining de- 

 velopment, reproduction, or growth. 



The second aspect of the subject is that which deals with the incor- 

 poration of new material in the cell and its subsequent distention by 

 an osmotic mechanism, upon the basis of researches of Pfeffer and 

 de Vries. The protoplast is dealt with as a sac. The products of the 

 metabolic processes converge in the vacuole, which in consequence 

 becomes the seat of osmotic forces and the center of the mechanism of 

 distention. An important feature of this scheme of operation is an 

 ideal "semi-permeable" membrane, not morphologically identifiable, 

 internal to the cellulose wall or other durable and visible integument. 

 The exploitation of the theory of permeability of this membrane has 

 been carried out in such manner as to place undue emphasis on the 

 action of the external layer of the protoplasm. The basic conception 

 of the diffusion of material into the vacuole remains sound, and the 

 differentiating action of protoplasm by which the increase in the con- 

 tents of the vacuole sets up an internal pressure expressed as turgidity 

 is undeniable. But the attempt to base all features of water-relations, 

 turgidity, and growth upon the action of solutions has been proved 

 inadequate and has resulted in an obvious neglect of the play of molec- 

 ular forces in surface tensions, in imbibition, and other activities of 

 matter in a colloidal state. 



The third group of inquiries has been directed toward measurement 

 for the purpose of establishing the physical constants of growth. 

 Auxesis or developmental enlargement in living things has been mis- 

 takenly dealt with as a unified process, or as a series of successive 

 reactions in studies of temperature effects, by many writers, and 

 coefficients of some apparent validity within a small part of the range 

 within which growth takes place have been found. Growth is a con- 

 stellation of activities and the rate of one of these dependent on tem- 

 perature may be the determining one when the particular process forms 

 either the retarding or leading agency. At other tunes the relative 



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