OCR Text |
Show 448 GENERAL RESULTS. CIIAP. XII. fl owers on the Same Plant' -and in this case a cross b etween the t wo Pla·n ts of Diob -italis• would . have done no goo d . B u t Seeds are often widely d1s1 orsocl by natura1 means, and one of the above two pla. nts or one of the ·u anc stors may have come f. rom a <.b stance, f rom a more sh ady Or sunny ' dry. o.r moist place, or .f rom a cl I. ~ue rent ki'nd of soil containing other organ.i c or · · atter vV e know from the ad1nuable 1 norgan1c m · . . * . . researc h es o f Messrs · Lawes and G1.l bert that dtfferent plants require and consume very different anl~unts of · ·c n1Inorganl atter· But the am.o unt in the soil would probably not 1nake so great a d1ff~rence to. tho several individuals of any particular species as migh~ at fi~·st b e exp ectecl ·, for the surrounding spec.i es. w1.th d1.u~e r· ent requirements would tend, fro1n exis.t ing. 1n greater or lesser numbers, to ke p eac~ spociCs 1n. a sort of equilibrium, with respect to what ~t could obtain from the soil. So it would be even with rcsp~ct to · sture d uri no- dry seasons ; and how powerful 1s the IDOl b · h 'l m· fiuence of a little more or l ss moisture In .t o fsto 1 on the presence and distribution of ~lants,. IS o ~n well shown in old pasture fi Ids which still retain trares of former ridg s and furrows. N e:vertholess, ~s the proportional numbers_ of the surrounding plants In two neighbouring places IS rarely xactly tho ~::nne, the . individuals of the same species will be snbJoetecl to somewhat different conditions with respect to what they can absorb from the soil. It is surprising how the f_ree growth of one set of plants affects others growJn~ mingled with them; I allowed the plants on rath~r . more than a square yard of turf which had boon clos~ Y mown for several years, to grow up ; and nino spcmos * , Journal of the Royal Agricult~ral Soc. of J~ngland,' vol. xxiv. part i. CHAP. XII. GENERAL RESULTS. 449 ou: of twenty were thus exterminated; but whether this was altogether due to the kincls h' h . bb' . w Ic grew up ro Ing the others of nutnnlent, I d o no t k now Seeds often lie dormant for several years . I. n t h e ground, and genninate when brought near the surface by any means, as by burrowing animals. They would probabl! be a~ecte~ by the mere circumst.ance of having long la~n dormant, for gardeners beheve that the production of double flowers and of fruit is thus infl~ enced. Seeds, m~reover, which were matured during drfferent seasons, w~ll have been subjected during the whole course of then development to different degrees of heat and moisture. I~ was sho_wn in the last chapter that pollen is often earned by Insects to a considerable ·distance from plant to plant. Therefore one of the parents or ancestors of our .two pJants of Digitalis may have been crossed by a d1stant plant growing under somewhat different conditions. Plants thus crossed often pro~ uce an unusu~lly la~ge number of seeds; a striking I~stance of ~h1s fact I~ afforded by the Bignonia, previ_ ously mentioned, wh1ch was fertilised by Fritz Muller With pollen from some adjoining plants and set hardly any seed, but when fertilised with pollen from a distant plant, was highly fertile. Seedlino·s from a cross of this kind grow with great vigour, andb transmit their vigour to their descendants. These, therefore, in the struggle for life, will generally beat and exterminate the seedlings from plants which have long grown near together under the same conditions, and will thus tend to spread . . When two varieties which present well-marked drfferences are crossed, their descendants in the later generations differ greatly from one another in external characters; and this is due to the augmentation 2 G |