INTERNAL FACTORS 179 



PART IV 

 REVIEW OF THE INTERNAL FACTORS DETERMINING SPECIFIC SHAPE 



SECTION 51. General. 



It may at once be admitted that our knowledge in this direction is 

 extremely vague. A point of some importance is, however, that even in 

 a completely homogeneous medium orienting stimuli of internal origin may 

 arise. For example, even in a radially constructed protoplast, lying in 

 water or in air, permanent differences may be maintained between the 

 external surface in contact with the medium and the internal layers. 

 Further, the absorption of oxygen, the evolution of carbon dioxide, and 

 other metabolic processes produce differences of concentration between 

 the interior and exterior, and steady inward and outward streams of 

 nutrient and excrete substances. These differences of concentration may 

 act as internal orienting stimuli just as do those in the external medium 

 which produce chemotropic reactions. 



The stimulatory action exercised by the surface-tension film may 

 together with other factors be responsible for the formation of the 

 living and irritable plasmatic membrane. Similarly the cell-wall formed 

 on the exterior of the protoplast is renewed when removed, and since this 

 occurs even when the protoplast is bathed by the cell-sap of the same 

 plant, it is evidently not the quality of the medium, but other factors which 

 are responsible for the non-formation of a cell-wall around the vacuole. 



Transpiration and its related processes favour the development of 

 cuticle, and the same factors or contact with air may induce the formation 

 of the conidia or sporangia of certain fungi. We are, however, as ignorant 

 of the connexion between the stimulus and response, as we are of the 

 causes which determine the differentiation of the peripheral layer of 

 meristem into specially shaped epidermal cells, or the reasons for the 

 dissimilar shape of the aerial and aquatic leaves of certain amphibious 

 plants. 



Whenever an organism modifies the external conditions locally or 

 generally by growing or moving from one medium to another, we are 

 dealing with an aitiogenic reaction in the results due to the change, just 

 as much as if the changes in the external conditions were produced 

 independently of the organism. In all cases, however, in which, the 

 eternal conditions remaining constant, a reaction is produced by a 

 rhange in the properties or irritability of the organism, this reaction is 

 lutogenic in character. An instance of the latter is afforded by a curvature 

 >f an organ produced owing to a change in its geotropic irritability, and a 

 irther one by the movements of naked protoplasts induced by changes of 



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