EMPLOYMENT OF CONSTRUCTIVE MATERIALS IN GROWING ORGANS. 357 



therefore no longer growing, are simply in outline and not shaded ; those which 

 are actively growing are shaded ; and the very slowly developing growing points 

 are represented black. Let us now assume that this diagrammatic plant has 

 completed its germination, and is already nourishing itself by means of assimila- 

 tion. The fully developed cotyledons c, which we suppose to contain chlorophyll, 

 and the fully developed leaf d, do duty as organs of assimilation. From these 

 the products of assimilation and metabolism have to travel on the one hand down 

 to the roots w w' w" , and also, and to a still greater extent, up to the young 

 parts of the shoot which are shaded dark. It is at once seen that between the 

 places where the consumption of material occurs, and the organs of assimilation, 

 parts are intercalated which are no longer growing, and thus do not use up the 

 materials. Nevertheless the plastic substances must travel through these fully de- 

 veloped parts to the (darkly shaded) growing portions. This takes place, so far 

 as the albuminous matters are concerned, in the phloem of _ the vascular bundles 

 which are indicated in the figure : starch, sugar, and asparagin, on the contrary, 

 travel in the layers of parenchyma, and, as micro-chemical investigations show, 

 chiefly in those layers which immediately surround the vascular bundles. This refers 

 especially to the transitory starch, as to the movement of which a few words more 

 may be added here. Where a young organ commences to grow, its small paren- 

 chyma-cells become filled with fine-grained starch, and then sugar appears in ad- 

 dition. As the part goes on growing, the starch and sugar increase ; they being 

 supplied in quantities greater than those consumed. Subsequently, the relation is re- 

 versed, and more is consumed than is supplied, and when the organ in question is 

 fully developed, the starch and sugar have disappeared. This takes place not 

 only when the supply of food is derived from the starch-forming organs of assimi- 

 lation, or from reservoirs of reserve-materials abounding in starch, but also when the 

 reservoirs of reserve-materials previously contained no starch at all, but cane-sugar 

 as in the root of the Beet, inulin as in the tubers of the Dahlia, or even fat as in 

 most germinating seeds, or, finally, cellulose as in the Date. In a word, apart 

 from rarer cases of plants in which the formation of starch is almost entirely 

 suppressed, as in the common Onion, the various kinds of substances which 

 form cell-walls, before they are directly made use of in growth, are at least in part 

 transformed into starch, which is then only used up directly as the cell-walls de- 

 velope. We have as yet no satisfactory explanation of this remarkable fact. Although 

 no doubt whatever now exists that the starch which enters the young parts of the 

 growing shoot (in a germinating Oak for instance, or the bulb of a Tulip just putting 

 forth shoots), and which at first increases but then always disappears at the conclusion 

 of the growth of the part, is derived from the starch contained in the reservoirs of 

 reserve-materials — the cotyledons or bulb-scales; nevertheless it is likewise certain, 

 on the other hand, that these small starch-granules are for and by themselves im- 

 movable, and that no single starch-granule is itself in the act of travelling. This 

 follows, on the one hand, from the fact that the transitory starch which has passed 

 into the growing parts consists of very small granules, whereas that in the reservoirs 

 of reserve-materials is large-grained and difi'erent in shape ; moreover it would 

 be quite impossible for solid starch-grains to pass through the closed cell-walls of the 

 j)arenchyma, while the necessary moving forces would also be wanting, ^^'hen in 



