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Show 190 EFFECT OF KILLING PRIMARY RADICLE. CHAP. III. the primary radicle or of. the lea< ling sho_ot. The followin()" considerations give us, as we uohevo, tho clue. Firstly, any cause which :listurbs tho constitution * is apt to induce revorswn ; such as the crossing of two distinct r.aces, ?r a change of conditions as when domestic ammals become feral. But the case which most concerns us, is the frequent appearance of peloric ~owors on the summit o~ a st.o~, or in the centre of the mfloresconco,-parts whiCh, 1t Is believed receive the most sap; for when an irregular flower b~comes perfectly regular or peloric, this may he attributed, at least partly, to reversion to a primi· tive and normal type. Even the position of a see~ at the end of the capsule sometimes gives to the seodlmg developed from it a tendency to rovc:-t. Secondly, reversions often occur by means of bu ls, mdeponclontly of reproduction by seed ; so that a bud may reve1:t to the character of a former state many bud-gonoratw~s ago. In the case of animals, r versions ma! occur m the individual with advancing ag . Thudly and lastly, radiclcs when they first protrude from the seed are always geotropic, and plumules or shoots almost always apogeotropic. If then any cause, such a~ an increased flow of sap or the presence of mycchum, disturbs the constitution of a ln.teral shoot or of. a secondary radicle, it is apt to revert to its primordi.al state· and it becomes either apogcotropic or gootr?pic, as th'e case may be, and consequently gr.o ws ther 01 vertically upwards or downwards. I t 1· s m· d ce d pos· * The facts on which the following conclui'ions are founded are given in 'The Vftriation of Animals and Plants under Domestication,' 2nd ctlit. 1875. On the cauHes leading to rev rsion see chap. xii. vol. ii. and p. 59, chap. xiv. On peloric flowers, chap. . .. u2 , aml soo ]J 3:-l7 on their Xlll. p. v ' . . With Po, ition ou tho plant. respect to see d s, P· 3'10 · On4 3r8e· version by meaus of buds, P· ' chap. xi. vol. i. CnAP.lli. SUlVIMAH. Y OF CIIAP'r !!JR. sible, or even probaulc, that this tendency to reversion may have been increased, as it is manifestly of service to the plant. SuMMAHY OF OnAPTEH. A part or organ may be callccl sensitive, when its 1rritation excites movement in an adjoining part. Now it has been shown in this chapter, that the tip of tho radicle of the bean is in this scnsQ sensitive to the contact of any small object attached to one side by shellac or gum-water; also to a slight touch with dry caustic, and to a thin slice cut off one side. The radicles of the pea were tried with attaehecl objects and caustic, both of which actcll. \Vith Phaseolus multijlorus the tip was hardly sensitive to small squares of attached card, but was sensitive to caustic and to s~i?ing. 'rhe radicles of Trop::colum were highly senSitive to contact; and so, as far as we could judge, were those of Gossypium herbaceum, and they were certainly sensitive to caustic. The tips of the radicles of Oucurbita ovifera were likewise hi()"hly sensitive to . b caustic, though only moderately so to contact. Ra-phanus sativus offered a somewhat doubtful case. Wi~h A'Jsculus the tips were quite indifferent to bodws attached to them tb.ouo·h sensitive to caustic 'rh ' b • . _ose of Quercus robur and Zea mays were highly sen- SitiVe to contact, as were the raclicles of the latter to caustic. In several of these cases the difference in sensitiven.ess of the tip to contact and to caustic was, as we beheve, merely apparent; for with Gossypium, Ra~hanus, and Cucurbita, the tip was so fine and · flexible that it was very difficult to attach any object to one of its sides. With the radiclcs of lEsculus the tips were not at all sensitive to small bodie~ attached to them; but it does not follow from this |