按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
parent； shows no superiority over self… fertilisation。） Darwin； however； considered it possible that this may sometimes be the case。 〃The sexual elements in the flowers on the same plant can rarely have been differentiated； though this is possible； as flower…buds are in one sense distinct individuals； sometimes varying and differing from one another in structure or constitution。〃 （〃Cross and Self fertilisation〃 （1st edition）； page 444。）
As regards the importance of this question from the point of view of the significance of cross…fertilisation in general； it may be noted that later observers have definitely discovered a difference between the results of autogamy and geitonogamy。 Gilley and Fruwirth found that in Brassica Napus； the length and weight of the fruits as also the total weight of the seeds in a single fruit were less in the case of autogamy than in geitonogamy。 With Sinapis alba a better crop of seeds was obtained after geitonogamy； and in the Sugar Beet the average weight of a fruit in the case of a self…fertilised plant was 0。009 gr。； from geitonogamy 0。012 gr。； and on cross…fertilisation 0。013 gr。
On the whole； however； the results of geitonogamy show that the favourable effects of cross…fertilisation do not depend simply on the fact that the pollen of one flower is conveyed to the stigma of another。 But the plants which are crossed must in some way be different。 If plants of Ipomoea purpurea （and Mimulus luteus） which have been self…fertilised for seven generations and grown under the same conditions of cultivation are crossed together； the plants so crossed would not be superior to the self… fertilised； on the other hand crossing with a fresh stock at once proves very advantageous。 The favourable effect of crossing is only apparent； therefore； if the parent plants are grown under different conditions or if they belong to different varieties。 〃It is really wonderful what an effect pollen from a distinct seedling plant； which has been exposed to different conditions of life； has on the offspring in comparison with pollen from the same flower or from a distinct individual； but which has been long subjected to the same conditions。 The subject bears on the very principle of life； which seems almost to require changes in the conditions。〃 （〃More Letters〃； Vol。 II。 page 406。）
The fertilitymeasured by the number or weight of the seeds produced by an equal number of plantsnoticed under different conditions of fertilisation may be quoted in illustration。
On crossing On crossing On self… with a fresh plants of the fertilisation stock same stock Mimuleus luteus （First and ninth generation） 100 4 3
Eschscholzia californica （second generation） 100 45 40
Dianthus caryophyllus （third and fourth generation） 100 45 33
Petunia violacea 100 54 46
Crossing under very similar conditions shows； therefore； that the difference between the sexual cells is smaller and thus the result of crossing is only slightly superior to that given by self…fertilisation。 Is； then； the favourable result of crossing with a foreign stock to be attributed to the fact that this belongs to another systematic entity or to the fact that the plants； though belonging to the same entity were exposed to different conditions？ This is a point on which further researches must be taken into account； especially since the analysis of the systematic entities has been much more thorough than formerly。 （In the case of garden plants； as Darwin to a large extent claimed； it is not easy to say whether two individuals really belong to the same variety； as they are usually of hybrid origin。 In some instances （Petunia； Iberis） the fresh stock employed by Darwin possessed flowers differing in colour from those of the plant crossed with it。） We know that most of Linneaus's species are compound species； frequently consisting of a very large number of smaller or elementary species formerly included under the comprehensive term varieties。 Hybridisation has in most cases affected our garden and cultivated plants so that they do not represent pure species but a mixture of species。
But this consideration has no essential bearing on Darwin's point of view； according to which the nature of the sexual cells is influenced by external conditions。 Even individuals growing close to one another are only apparently exposed to identical conditions。 Their sexual cells may therefore be differently influenced and thus give favourable results on crossing； as 〃the benefits which so generally follow from a cross between two plants apparently depend on the two differing somewhat in constitution or character。〃 As a matter of fact we are familiar with a large number of cases in which the condition of the reproductive organs is influenced by external conditions。 Darwin has himself demonstrated this for self…sterile plants； that is plants in which self…fertilisation produces no result。 This self…sterility is affected by climatic conditions： thus in Brazil Eschscholzia californica is absolutely sterile to the pollen of its own flowers； the descendants of Brazilian plants in Darwin's cultures were partially self…fertile in one generation and in a second generation still more so。 If one has any doubt in this case whether it is a question of the condition of the style and stigma； which possibly prevents the entrance of the pollen…tube or even its development； rather than that of the actual sexual cells； in other cases there is no doubt that an influence is exerted on the latter。
Janczewski （Janczewski； 〃Sur les antheres steriles des Groseilliers〃； 〃Bull。 de l'acad。 des sciences de Cracovie〃； June； 1908。） has recently shown that species of Ribes cultivated under unnatural conditions frequently produce a mixed （i。e。 partly useless） or completely sterile pollen； precisely as happens with hybrids。 There are； therefore； substantial reasons for the conclusion that conditions of life exert an influence on the sexual cells。 〃Thus the proposition that the benefit from cross…fertilisation depends on the plants which are crossed having been subjected during previous generations to somewhat different conditions； or to their having varied from some unknown cause as if they had been thus subjected； is securely fortified on all sides。〃 （〃Cross and Self fertilisation〃 （1st edition）； page 444。）
We thus obtain an insight into the significance of sexuality。 If an occasional and slight alteration in the conditions under which plants and animals live is beneficial （Reasons for this are given by Darwin in 〃Variation under Domestication〃 （2nd edition）； Vol。 II。 page 127。）； crossing between organisms which have been exposed to different conditions becomes still more advantageous。 The entire constitution is in this way influenced from the beginning； at a time when the whole organisation is in a highly plastic state。 The total life…energy； so to speak； is increased； a gain which is not produced by asexual reproduction or by the un