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Show 10 INTRODUCTORY REMARKS. CHAP. I. Mi·mulus luteus and Ipomwa purpurea, both of which, unlike the Linaria and Dianthus, are highly s Iffertile if insects are excluded. Some flowers on a single plant of both species were f rtilised with th ir own pollen, and others were crossed with pollen from a distinct individual; both plants being prot ct d by a net from insects. The crossed and self-f rtili a seed thus produced were sown on opposite sides of th sam pots, and treated in all respects alike ; and the plants when fully grown were measured and com par l. With both species, as in the cas s of the Linaria ancl Dianthus, the crossed seedlings were conspi ·nou~ly superior in height and in other ways to tho sel ffertilised. I therefore detennined to b gin a long series of experiments with various plants, and th were continued fo1· the following eleven y ars; ana w shall see that in a large majority of cases th eros d beat the self-fertilised plants. Several of the e .. rceptional cases, moreover, in which the cross d plants were not victorious, can be explained. It should be observed that I have spoken for th sake of brevity, and shall continue to do so, of crossed and self-fertilised seeds, seedlings, or plant ; these terms implying that they are the product of cross d or self-fertilised flowers. Cross-fertilisation always n1eans a cross between distinct plants which were rai eel fron1 seeds and not from cuttings or buds. Self-fertilisation always implies that the flowers in question w re impregnated with their own pollen. ~y experime~t~ were tried in the foll wing manner. A Single plant, If 1t produced a sufficiency of flowers, or two or three plants were placed under a net stretched on a frame? and large enough to cover the plant (toget.her ~Ith the pot, when one was used) without touching 1t. This latter point is important, for if CHAP. I. INTRODUCTORY REMARKS. 11 the flowers touch the net they may be cross-fertilised by bees, as I have known to happen; and when the net is wet the pollen may be injured. I used at first "white cotton net," with very :fine meshes, but afterwards a kind of net with meshes one-tenth of an inch in diam.eter ; and this I found by experience effectually excluded all insects excepting Thrips, which no net will exclude. On the plants thus protected several flowers were marked, and were fertilised with their own pollen ; and an equal n urn ber on the same plants, marked in a different manner, were at the same time crossed with pollen from a distinct plant. The crossed flowers were never castrated, in order to make the experiments as like as possible to what occurs under nature with pla:nts fertilised by the aid of insects. Therefore, some of the flowers which were crossed may have failed to be thu:::~ fertilised, and afterwards have been self-fertilised. But this and some other sources of error will presently be discussed. In some few cases of spontaneously self-fertile species, the flowers were allowed to fertilise themselves under the net; and in still fewer cases uncovered plants were allowed to be freely crossed by the insects which incessantly visited them. There are some great advantages and some disadvantages in my having occasionally varied my method of proceeding; but when there was any difference in the treatment, it is always so stated under the head of each species. Care was taken that the seeds were thorough! y ripened before being gathered. Afterwards the crossed and self-fertilised seeds were in most cases placed on damp sand on opposite sides of a glass tumbler covered by a glass plate, with a partition between the two lots ; and the glass was placed on the chimney-piece in a warm room. I could thus observe the germination of |