352 DIRECTION OF PLUMULE AND EADICLE. 



gradually absorbed like the perisperm. In the former they assume 



a more or less leafy aspect, exercis- 

 ing the functions of leaves for a 

 certain period, and ultimately decay- 

 ing. While the radicle descends 

 towards the centre of the earth, pro- 

 ducing roots of a pale colour, the 

 plumule has a tendency to ascend, 

 forming the leafy axis, and assuming 

 a green colour under the influence of 

 light and air. 



Direction of Plumule and Hadicle. — Various attempts have 

 been made to explain the ascent of the plumule and the descent of 

 the radicle, but none of them are satisfactory. Physiologists have 

 not been able to detect any law to which they can refer the phenomena, 

 although certain agencies are obviously concerned in the effects. 

 Some have said that the root is especially influenced by the attraction 

 of the earth, while the stem is influenced by light. Experiments 

 have shown that the direction of the root is not owing.to the moisture 

 of the soil, and that the ascent of the stem is not due to the action of 

 light and air; for roots descend, and stems ascend, even when the 

 latter are placed in contact with the earth, and the former submitted 

 to the action of light. Knight thinks that the direction of stem and 

 roots may be traced to gravitation, and the state of the tissues. When 

 a branch is horizontal, the fluids gravitate towards the lower side ; a 

 vigorous growth takes place there; the tissues enlarge, and, by 

 increasing more than those on the upper side, an incurvation is pro- 

 duced, the convexity of which looks downwards, and thus the extremity 

 of the branch is directed upwards. Again, in the root the increase 

 takes place by the extremity, and the fluids by their gravity cause 

 this to retain always a descending direction. A similar explanation 

 is given by Dodart. Dutrochet refers the phenomena to endosmose, 

 which varies in its effects according to the comparative size of the 

 cells in the centre and circumference of an axis. In young stems 

 with large pith, the central cells are larger, and they diminish towards 

 the circumference ; whereas in roots, according to him, the diminution 

 takes place in the reverse manner. Large cells distend more rapidly 

 than smaU ones ; and, according to their position in the axis, wiU 



Fig. 625. Germination of the dicotyledonous aperispermic seed of Acacia Julibrissin. 

 e, Spermoderrn or testa, r. Radicle of the embryo.* t, Tigellus or cauliculus. c. Cotyledons. 

 g, Gemmule or plumule. 1, First stage : in which the radicle ruptures the envelope or 

 spermoderrn, and appears externally at the micropyle. 2, Second stage : where the parts 

 of the embi-yo are further disengaged from the covering, the summit of the cotyledons only 

 being retained by the spermoderrn. 3, Third stage: where the embryo is entirely dis- 

 engaged from the envelope or spermoderrn, and the cotyledons, c c, are separated so as to 

 exhibit the plumule, g. 



