FLORA AND SYLVA. 



IS DIFFERENTIATION, OR DIFFERENCE IN CONSTITUTION, IN 

 FLOWERS NECESSARY FOR THEIR COMPLETE FERTILITY ? 



" There is the clearest evidence," Darwin ever happened to be most in its prime that 



says, " that the advantage of a cross depends would exercise a prepotent influence in the 



wholly on the plants differing somewhat in fertilization. But the flowers fertilized with 



constitution, and that the disadvantages of their own pollen had no other pollen but their 



self-fertilization depend on the two parents, own to depend upon, and that developed under 



which have the stamens and pistils combined the net, which must fertilize them, or none at 



in the same flower, having a closely similar all. 



constitution " (" Cross- and Self-Fertilization It is a well-known and well-established 



of Plants," p. 254). To these conclusions fact, from thousands of observations amongst 



Darwin was led by the method which he farmers and gardeners gener- 



adopted in his experiments with flowers. shade°* ally, that flowers grown under 



The flowers on which he experimented shade have their productive 



were placed under a close-meshed net. The capacity very much impaired, and sometimes 



net necessarily deprived the entirely obliterated. Darwin consequently 



D method flowers under it of the full in- found that the flowers self-fertilized under the 



fluence of the solar rays and net, either with their own pollen or with that 



wind. It would consequently affect the full of the similarly covered flowers on the same 



ripening of the pollen in flowers grown under plant, produced seeds inferior, oftentimes in 



such a condition. " My experiments," Dar- number as well as in quality — as evidenced 



win says, " were tried in the following man- in the inferior size and vigour of the seedlings 



ner : A single plant, if it produced a suffi- grown from such seeds — to those of flowers 



ciency of flowers, or two or three plants were on the same plant crossed from the pollen of 



placed under a net stretched on a frame and a plant grown naturally outside the net. 



large enough to cover the plant without We here give in illustration of Darwin's 



touching it. On the flowers thus protected method his experiment with the Foxglove 



several flowers were marked and were ferti- recorded on pp. 452, and 82, 83, of " Cross- 



lized with their own pollen, and an equal and Self-Fertilization of Plants." 



number on the same plants were at the same Darwin experimented with two plants of 



time crossed with pollen from a distinct plant, the wild Foxglove {Digitalis purpurea) grow- 



The crossed flowers had not their anthers re- ing closely adjoining each other in the same 



moved" (ib., pp. 10, 11). clump. One of the plants he covered with a 



In these experiments, consequently, the net, and when the flowers expanded he ferti- 



crossed flowers had a great advantage. The lized some of the flowers of the covered plant 



self-fertilized flowers had only their own with pollen from their own flower, or from 



pollen, and that developed under the net, to other flowers on the same plant ; other flowers 



fertilize them, but the crossed flowers had not on the covered plant he fertilized with pollen 



only their own pollen — for, as we have seen from the uncovered plant. As the two plants 



above, their anthers were not removed — but were growing almost side by side, the difficulty 



pollen from another plant applied to them as necessarily arose how could " two or more 



well, and that too grown naturally outside the net; plants growing close together, either in their 



for Darwin wished, by leaving the flowers own native country or in a garden, be differ- 



their own pollen, and at the same time cross- ently acted on, inasmuch as they appear to 



ing them with other pollen, " to make the be exposed to exactly the same conditions " 



experiments as like as possible to what occurs (p. 45 2). The results and conclusions at which 



under Nature with plants fertilized by the aid Darwin arrived from this experiment we re- 



of insects." cord in his own words. "Some flowers on a 



The cross-fertilized had consequently two wild Foxglove," he says, " were self-fertilized 



sets of pollen to choose between, and which- and others were crossed from pollen from an- 



