298 STAGES OF GERMINATION. 



ruptures the integument, and is nourished at the expense of the 

 cotyledons, which are often fleshy, containing much starchy matter, as 

 in the Bean and Pea, along with oily matter, as in the Nut and Rape 

 seed. There are thus two stages of germination that in which the 

 embryo undergoes certain changes within the seed itself, and that in 

 which it protrudes through the integuments and becomes an indepen- 

 dent plant. 



623. The embryo then, nourished at the expense of its perisperm 

 and cotyledons, continues to grow, and usually protrudes its radicular 

 extremity (fig. 526, 1) in the first instance, which is nearest the sur- 

 face, and next the micropyle. This, which in the embryo is very short, 

 and confounded with the cauliculus so as to form the first internode, 

 becomes thickened by addition to its extremity (fig. 526, 2), and the 

 division between the ascending and descending axis becomes more 

 marked. The caulicule or axis also elongates, bearing at its summit 

 the plumule, which now appears outside the integuments (fig. 526, 3, 



g), forming the second internode, either 

 accompanied by the cotyledons, or 

 leaving them still within the seed coats. 

 In the latter case, the cotyledons are 

 usually fleshy and of a pale colour, 

 and become gradually absorbed like 

 the perisperm. In the former they 

 assume a more or less leafy aspect, 

 exercising the functions of leaves for a 

 certain period, and ultimately decaying. While the radicle descends 

 towards the centre of the earth, producing roots of a pale colour, the 

 plumule has a tendency to ascend, forming the leafy axis, and assum- 

 ing a green colour under the influence of light and air. 



624. Direction of Plumule and Radicle. 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 cer- 

 tain agencies are obviously concerned hi 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 



Fig. 526. Germination of the dicotyledonous aperispermic seed of Acacia Julibrissin. , 

 Spermoderm or testa, r, Radicle of the embryo, t, Tigellus or cauliculus. c, Cotyledons, g, 

 Gemmule or plumule. L First stage: in which the radicle ruptures the envelope or spermoderm, 

 and appears externally at the micropyle. 2. Second stage: where the parts of the embryo are 

 further disengaged from the covering, the summit of the cotyledons only being retained by the 

 spermoderm. 3. Third stage : where the embryo is entirely disengaged from the envelope or 

 spermoderm, and the cotyledons, c c, are separated so as to exhibit the plumule, g. 



