250 



METAMORPHOSIS 



so that we may pass from Basidiobolus directly to our third and last example 

 the flowering plant. In its earliest stage the flowering plant is nothing more 

 than a single cell (the ovum), similar in all respects to the cell of a fungus. 

 In this case also the cell grows and divides, but all the daughter-cells do not 

 fulfil the same function nor do they continue to have the same structure ; they 

 are no longer of equal value and must remain connected if they are to continue 

 in existence. It is not the cells of which the higher plant is composed that 

 attract our attention in the first instance but rather the external members of 

 the plant ; the cell structure of each individual part is revealed on subsequent 

 microscopical investigation. The external segmentation of the plant is, however, 

 already suggested in the embryo whilst still within the seed, exhibiting as 

 it does two poles, one of which we term the radicle, the other the plumule. 

 The distinction between them exhibits itself most clearly during germination 

 by the fact that the root on leaving the seed pushes its way downwards 

 into the soil while the plumule grows in the opposite direction. A careful 

 study of the conditions under which the differentiation of these two poles arises 

 forces us to the conclusion that we must seek its cause not in external factors, 

 such as play so important a part in the morphology of Basidiobolus, but in some 

 internal factors. The further progress of development in a flowering plant 

 also convinces us that internal causes play a most important part, for though 

 the external factors remain completely unchanged the plant itself alters in shape 

 from day to day. The root grows apically and increases in length in a well- 

 marked manner ; it forms lateral branches which resemble the principal root 

 in all essential respects. The plumule divides into axis and leaves and gives 

 origin to lateral axes, also provided with leaves. The leaves may be of diverse 

 shape, but usually agree in exhibiting vigorous unilateral growth, forming thin 

 but at the same time broad surfaces, green in colour, owing to the chlorophyll 

 contained in their cells. They change their shape several times in the course of 

 the vegetative period ; the typical foliage leaves are preceded by simple leaf -like 

 organs, and are followed by them also ; the branch produces alternately scale 

 and foliage leaves, or at the apex proceeds to form bracts, and finally flowers. In 

 the flowers are developed the organs of sexual reproduction, by whose agency 

 new plants are formed. In addition to sexual organs we find in thousands 

 of flowering plants organs of vegetative reproduction as well. 



The flowering plant passes through a ' life cycle ' which, at least in certain 

 cases, begins with the germination of the seed and ends with its formation, 

 and which extends over one or more seasons. We found that in Basidiobolus the 

 various growth forms were mainly dependent on external conditions, but in the 

 flowering plant we have something entirely different, namely the intimate 

 connexion between the functions of the various members and their external 

 and internal structure. We have seen, in Part I, how all the parts of the 

 higher plant no longer fulfil the same functions, as they do in unicellular 

 forms. The root, we have found, is the organ for absorption of water and 

 soluble salts from the soil, the roothairs being specially concerned in this 

 absorption. There can be no doubt that these roothairs, by vastly increasing 

 the surface of the root, by their intimate union with the soil particles, and 

 by their excretion of certain substances, are specially adapted for this purpose. 

 The substances acquired in this way are, in part, transferred to or elaborated 

 in the aerial parts of the plant, and hence we find special conducting strands 

 developed in the interior of the root which are in intimate connexion with 

 similar conducting channels in the shoot. The chief of these are the tracheae, 

 cells which have lost their protoplasmic contents and consist of empty tubes 

 into which the water flows. 



The function of the leaf is entirely different. It is the carrier of the chloro- 

 phyll by means of which carbohydrates are formed from carbon-dioxide. For 

 this purpose, as we have already seen, sunlight is essential. The chlorophyll has 



