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LECTURE XII 



THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 



THE DISSOLUTION OF STARCH IN SEEDS BY MEANS OF ENZYMES 



Chlorophyll is not developed in all the cells of the higher plant ; in 

 the petiole and most stems it is small in amount as compared with the colour- 

 less protoplasm, while subterranean parts, such as roots, rhizomes, &c., are 

 quite free from it. Cells without chlorophyll, however, as we have seen, are 

 entirely incapable of forming carbohydrates from carbon-dioxide, and conse- 

 quently they are forced to obtain it from green cells. It necessarily follows 

 that the products of assimilation in the leaf blade must migrate from it to 

 undergo conversion elsewhere. This is probably true only of the carbo- 

 hydrates, since, to all appearance, with their aid, cells free from chlorophyll are 

 also capable of forming proteids in the dark. Should proteids also arise in 

 the leaf to any great extent they too probably migrate. Migration of the pro- 

 ducts of assimilation out of the leaf may also be deduced from other evidence. 

 Leaves in their earliest stages of development are colourless, and consequently 

 must depend for their further growth on external supplies of organic material ; 

 at a later date, after the formation of chlorophyll, they begin to exhibit the 

 phenomenon of carbon assimilation, and we must conclude that they employ 

 the products of that assimilation in the first instance for their own construction. 

 After a short time, however, a stage in development is reached when the leaf 

 has attained its definite size, and then arises the question, what becomes of 

 the products if they be not translocated ? Experience teaches us that accumu- 

 lation of starch in the confined limits of the chloroplast limits its assimilatory 

 activity, but, as a matter of fact, such an injurious accumulation of starch 

 does not take place under ordinary circumstances, because any excess under- 

 goes translocation. We shall find in the next lecture that a leaf filled with 

 starch may often lose it all in the course of a single night, and the fact that in 

 isolated leaves no such disappearance of starch takes place demonstrates that this 

 disappearance in the normal leaf is due not to a consumption of the starch 

 in situ, but to its transference from the leaf to the stem by way of the petiole. 



Starch is, however, solid, insoluble and incapable of migrating either actively 

 or passively from cell to cell. Its translocation is possible only in the form of 

 a soluble carbohydrate, after undergoing chemical alteration. There is no fact 

 more clearly established in vegetable physiology than this,that a supply of soluble 

 organic plasta passes from the assimilating leaf to regions of the plant which 

 have themselves no power of bringing about carbon assimilation. A study of the 

 transformations which these organic substances undergo, reveals to us certain 

 functions performed by them, of which the following are the most important : — 



1. The products of assimilation in the leaf blade act as constructive mate- 

 rials ; they are transferred to wherever the plant is using these bodies — to 

 the growing points of the stem and root and also to the cambium. In these 

 situations, the organic materials manufactured by the leaf are devoted to the 

 formation of new cells. 



2. The products of assimilation act as reserve substances, either where they 

 are formed or, after translocation, are stored up more or less permanently in 

 other situations. Such reserves are afterwards converted into plasta and 

 employed as constructive materials or for other purposes. 



3. The products of assimilation are oxidized, and in consequence become 

 once more altered into simple inorganic bodies such as are used by the leaf in 

 the manufacture of organic compounds. Katabolic processes, such as these, 

 are inseparable from all vital activities. Those substances which are sacrificed 

 in this way may be termed working materials. 



4. Since the conversion of materials is accompanied by translocation we 



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