THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 173 



quite definite regions of transformation and others which might be considered 

 as supplying their wants. The demand for materials is most prominent in all 

 growing points. In these regions, it is true, no great quantity of materials iscalled 

 for at any one time, but, since a continual formation of new cells takes place 

 in these places, substances needed for the formation of cell-walls, of protoplasm, 

 and of osmotic substances are required almost the whole year round. In a tree, 

 as soon as the extension of the shoots of the present year is completed, frequently 

 rapidly and always at the expense of last year's materials, early in the year the 

 primordia of the next year's buds are laid down, the development of which 

 slowly progresses during the winter. Besides, the cambium is also active, and 

 by its constant production of wood and bast, demands a continuous supply of 

 nutritive materials. After the formation of the flowers comes the construction 

 of fruit and seed, and finally the accumulation of stores in the root and stem, 

 beginning at the base and gradually extending upwards. In all cases we 

 recognize as migratory materials the substances so often mentioned, sugar, 

 proteid, and amides, and further, we see that reserves are formed from them, 

 either temporary in their nature or destined to remain quiescent for longer 

 periods. Trees are distinguished from annuals inasmuch as the latter store 

 reserves permanently in their seeds only, while the distinction between trees and 

 perennial herbaceous plants lies in this, that the latter deposit their reserves 

 not in aerial but in subterranean storehouses. 



A transference of material takes place normally from all storehouses of 

 reserve, but the plant is, however, able under abnormal conditions to permit 

 of consumption and transference also taking place in other than the normal situa- 

 tions. For example, if we remove the growing points and thus render their 

 development impossible, other appropriate organs may become centres of 

 consumption (compare VOCHTING'S experiments, Lecture XXVI), and if we 

 allow the growing points to develop with insufficient food supplies, materials are 

 drawn from older parts of the plant and in such quantities that these older organs 

 die off. In cultures carried out in darkness one often notices apical growth 

 proceeding at the cost of the older and moribund leaves. 



We have still, in conclusion, and as an appendix to our treatment of the 

 subject of alteration and migration of materials, to note the changes which 

 these moving plasta undergo when they reach their destinations. These changes 

 are most varied, especially when the plasta are altered into constructive mate- 

 rials. We have to compare only such relatively simple migratory bodies as 

 soluble carbohydrates, amides, and minerals with the complicated structure 

 of the cells made from them. As to these metamorphoses of materials we are 

 still very much in the dark. The processes by which reserves are formed from 

 translocatory materials are better understood, since these become transformed, 

 on the whole, into the same bodies as those from which they were derived. 

 Here, also, however, we come face to face with many debatable points. 

 Although, for example, starch or reserve cellulose is formed from glucose 

 we are ignorant of the immediate conditions of the transformation ; we can 

 only say that the changes are not very extensive, and it is only a question 

 of time before we shall be acquainted with all the chemical details of the 

 process. The matter is not nearly so simple with proteid. This sub- 

 stance, as we have seen, breaks down into bodies whose constitution is very 

 different from its own. If germination takes place in light no noticeable 

 accumulation of amides takes place, because they are at once retransformed 

 into proteids at the regions where consumption is going on. But if we allow 

 the seeds to germinate in the dark these bodies accumulate in such quantities 

 that we can demonstrate their crystals with the greatest readiness under the 

 microscope, after precipitation with alcohol. Obviously the conditions for 

 the reformation of proteid are not fulfilled in darkness, and hence a culture in 

 the dark is always employed when it is desired to obtain amide bodies in large 



