SELF-REGULATION 5 ! 5 



occurs. Since the needs of an organism may vary within wide limits, 

 it is essential that the production of a particular substance should be 

 regulated according to the amount present, and that the requirements of 

 an organism should act as a stimulus exciting a sufficiently increased 

 production to satisfy them. Many examples of such regulatory changes 

 have already been given (cf. Sect. 77), others are afforded by the regulation 

 of turgor in growing organs, and by the storage of reserve-materials, for these 

 do not accumulate beyond a certain limit however abundant the food-supply 

 may be. Similarly the mobilization of reserve-food-materials is regulated 

 by the amount consumed, and the direction of translocation is determined by 

 the localization of the consuming organs (Sect. 108). Thus when a tree 

 buds out afresh after premature defoliation, nutrient materials travel to the 

 growing regions from far distant storage- receptacles, where they would 

 otherwise have remained intact until the following spring. In all such cases 

 it has to be determined whether the commencement of the mobilizing 

 metamorphoses, was due to the removal of the traces of the diosmosing 

 products previously present, and its continuance to their rapid consump- 

 tion as fast as they are formed, or whether the consuming cells exert 

 some special vitalistic or other influence upon those in which reserve-food 

 is stored. Certain recent researches by Hansteen and Puriewitsch * show- 

 that the latter is not necessarily essential, for by the continual removal 

 of the diosmosing sugar it was found possible to empty all the starch from 

 the isolated endosperm of grasses, and from the cotyledons of Phaseolus, 

 as well as to remove all the glucose from single bulb-scales of A Ilium cepa. 

 The removal of starch ceases in a 2 to 3 per cent, solution of sugar, and 

 hence the conversion of starch soon stops when the gypsum cast in which 

 the experimental object is partially embedded stands in only a small 

 quantity of water. Puriewitsch found that if more sugar was added, the 

 endosperm, cotyledons, or bulb-scales might even become refilled with 

 starch or glucose as the case might be. Similarly the chloroplastids of 

 many plants form starch when fed with sugar or glycerine, but temporarily 

 lose the power of assimilating carbon dioxide when the cell is overloaded 

 with assimilatory products (Sect. 55). This may, however, be avoided for a 

 time by the deposition of the soluble products in the form of starch, or by 

 their removal to neighbouring cells. 



A nutrient substance may be maintained intact not only by the 

 cessation of the functional activity by which it is consumed, but also 

 by the formation of other substances which can replace it and can enter 

 more readily into metabolism. This question has already been discussed 

 (Sect. 67), and it has been shown that the phenomena of selection and 



1 Hansteen, Flora, 1894, Erg.-bd., p. 419; Puriewitsch, Bar. d. Bot. Ges., 1896, p. 207; Jahrb. 

 f. wiss. Bot., 1897, Bd. xxxi, p. i. Cf. Sect. 109. 



Ll 2 



