528 LECTURE XXXI. 



on growth, and at the same time we have had to consider the relations of direction 

 and position under which the embryonic rudiments of new organs arise, and of 

 the external and internal symmetry of their organisation. These relations are 

 possible simply because gravitation and light, according to their nature, act on 

 the plastic substance of the plant in definite directions; and we shall have 

 opportunities subsequently, when considering geotropism and heliotropism, of 

 becoming acquainted with other effects of these two natural forces, where it is also 

 a matter of effects appearing in definite directions, but of effects on organisations 

 already more or less fully developed, whereas we are here concerned exclusively with 

 the effects of external forces on the developing organisation itself. 



Having thus become acquainted with the influence of gravitation and light on 

 the embryonic rudiments of the organs, by means of a few examples, I now proceed 

 to show how the same forces co-operate during the further growth of organs already 

 established, or, as we may say generally, how they affect post-embryonic growth. 



2. Post-embryonic relations of configuration. In this case also, as will be shown 

 further on, the influence on growing organs in a definite direction may still come into 

 consideration ; but, in the majority of the known cases, the effects of gravitation and 

 light here present themselves in far more complicated forms, indeed so that the whole 

 physiological quality of an organ may be determined by them. 



In this sense the behaviour of the subterranean shoots of Draccena and Yucca 

 will at once illustrate what is meant \ These well-known large Liliacece develope from 

 the base of their upright stems root-like shoots of a thickness varying between that 

 of the thumb and that of the arm, which grow down vertically into the earth, and in 

 their turn produce numerous long roots which feed the whole of the large plant, 

 but remain thin and filiform. These subterranean shoots produce, it is true, 

 numerous leaves at their broad growing-points, but these leaves remain rudimentary 

 and form annular thin membranes. So long as the plant remains undisturbed, 

 proper foliage leaves never arise from the growing-points of these subterranean 

 shoots. Their development can be effected, however, simply by completely inverting 

 such a plant placed on a support, as in Fig. 344 h g, and directing the root-system/ 

 contained in the flower-pot upwards. The growth of the now down-turned bud is 

 injured if it has no opportunity of curving upwards ; the development of the growing- 

 points on the thick rhizome shoot a d, however, is promoted. The leaves produced 

 here develope henceforward no longer as annular membranes, but in the form of 

 foliage-leaves, finally coming above the soil, and thus leaf-shoots arise directly from 

 the up-turned rhizomes. It is evident that in the experiment nothing whatever 

 has been altered except the direction in which gravitation acts on the plant. The 

 rhizomes in the opaque soil are inaccessible to the light both before and after, the 

 relations of moisture in the surrounding soil are the same as before, and nevertheless 

 the post-embryonic development of the leaves arising at the growing-point are forth- 

 with altered : instead of so-called cataphyllary leaves or scales, vigorous foliage- 

 leaves are produced, which, however, of course only become green and flat when 

 they grow forth above the soil. Moreover, it is possible to make the experiment 



' What is said in the text as to Yucca and Draccena (sub-genus Cordyline) is described more 

 in detail in my treatise ' Ubc>- Stoff und Form ' (Arb. des bot. Inst, in Wzbg. B. II, pp. 475, &c.). 



