460 METABOLISM IN LIVING PLANTS. 



remain in seeds unaltered for years, and as if dead; but if the seed germinates, and 

 the seedling begins to develop, the starch is dissolved, that is to say, becomes changed 

 into another carbohydrate, and finally is made use of in the construction of the cell- 

 walls of the growing seedling by a fresh transformation. Starch-grains in various 

 species of plants differ very much in size as well as in shape. The largest grains 

 exhibit under the microscope alternating blue and red zones, which are accounted 

 for by the difl^erence in the amount of water contained in the several zones. The 

 bluish zones contain less, and the red-tinted more water. Many starch-grains exhibit 

 a " nucleus " or hilum which is rich in water, and which is situated excentrically 

 in the grains of the Potato and of Canna (fig. 124^); centrally in those of the 

 Wheat. A space may be present instead of the hilum, as in the starch-grains of 

 beans and other pulses (fig. 124*), in consequence of the drying up of the substance 

 of the hilum. In most plants the starch-grains have a rounded form; but those of 

 the Corn-cockle (Agrostemma Githago) are fusiform and club-shaped (fig. 124^). 

 Those of species of Euphorbia resemble tiny bones (fig. 124^), and others again are 

 angular and cornered like crystalline figures (figs. 124^ and 124^'). This last form 

 is seen especially when the cells which serve as store-houses are densely crowded 

 with starch-grains so that growth becomes checked, and a mutual flattening takes 

 place. In the Oat and Rice many small angular starch -granules are cemented 

 together to form ellipsoidal grains (figs. 124 * and 124-'''), and in the starch from the 

 corm of the Meadow Safiron, regular groups of four rounded grains, each exhibiting 

 a hollow hilum, are found (fig. 124^^). Granulose forms the chief of the two 

 carbohydrates which are intimately mixed to form starch. It is soluble in saliva, 

 and is therefore extracted by it, while the cellulose remains behind insoluble, a fact 

 which is of great importance with regard to the digestibility of the starch present 

 in such abundance in flour and bread. 



In close connection with these essential building materials are other substances 

 which, though not themselves serving as building materials, take an active part in 

 their production. These furnish the conditions under which the manufacture and 

 transport of the building substances, and the growth and propagation of the plants 

 can occur. They avert injurious influences, regulate light and heat, and are of use 

 to the plant in a hundred minor directions. 



To these substances, which may be termed accessory, belong, first of all, the 

 pigments chlorophyll, phycoerythrin, and anthocyanin, which are so important on 

 account of their relations to light and heat, and whose role has already been alluded 

 to. Then we have those compounds whose function is to allure animals to the 

 plants in order to bring about fertilization or the distribution of the seeds and 

 spores, or whose significance lies in the fact that they frighten and ward off animals 

 which might be injurious to the plants. In this connection are of course to be 

 mentioned colouring-matters which are formed in flowers and fruits in order that 

 these may be rendered visible at a distance to those animals whose visits benefit the 

 plant: first of all, anthocyanin, which in the presence of acids is red, but otherwise 

 appears violet or blue; and then anthoxanthin, to which most yellow flowers and 



