sation possible。 His view that cleistogamous flowers are derived from originally chasmogamous flowers has been confirmed by more recent researches。 Conditions of nutrition in the broader sense are the factors which determine whether chasmogamous or cleistogamous flowers are produced； assuming； of course； that the plants in question have the power of developing both forms of flower。 The former may fail to appear for some time； but are eventually developed under favourable conditions of nourishment。 The belief of many authors that there are plants with only cleistogamous flowers cannot therefore be accepted as authoritative without thorough experimental proof； as we are concerned with extra…european plants for which it is often difficult to provide appropriate conditions in cultivation。
Darwin sees in cleistogamous flowers an adaptation to a good supply of seeds with a small expenditure of material； while chasmogamous flowers of the same species are usually cross…fertilised and 〃their offspring will thus be invigorated； as we may infer from a wide…spread analogy。〃 （〃Forms of Flowers〃 （1st edition）； page 341。） Direct proof in support of this has hitherto been supplied in a few cases only； we shall often find that the example set by Darwin in solving such problems as these by laborious experiment has unfortunately been little imitated。
Another chapter of this book treats of the distribution of the sexes in polygamous； dioecious； and gyno…dioecious plants （the last term； now in common use； we owe to Darwin）。 It contains a number of important facts and discussions and has inspired the experimental researches of Correns and others。
The most important of Darwin's work on floral biology is； however； that on cross and self…fertilisation； chiefly because it states the results of experimental investigations extending over many years。 Only such experiments； as we have pointed out； could determine whether cross… fertilisation is in itself beneficial； and self…fertilisation on the other hand injurious； a conclusion which a merely comparative examination of pollination…mechanisms renders in the highest degree probable。 Later floral biologists have unfortunately almost entirely confined themselves to observations on floral mechanisms。 But there is little more to be gained by this kind of work than an assumption long ago made by C。K。 Sprengel that 〃very many flowers have the sexes separate and probably at least as many hermaphrodite flowers are dichogamous； it would thus appear that Nature was unwilling that any flower should be fertilised by its own pollen。〃
It was an accidental observation which inspired Darwin's experiments on the effect of cross and self…fertilisation。 Plants of Linaria vulgaris were grown in two adjacent beds； in the one were plants produced by cross… fertilisation； that is； from seeds obtained after fertilisation by pollen of another plant of the same species； in the other grew plants produced by self…fertilisation； that is from seed produced as the result of pollination of the same flower。 The first were obviously superior to the latter。
Darwin was surprised by this observation； as he had expected a prejudicial influence of self…fertilisation to manifest itself after a series of generations： 〃I always supposed until lately that no evil effects would be visible until after several generations of self…fertilisation； but now I see that one generation sometimes suffices and the existence of dimorphic plants and all the wonderful contrivances of orchids are quite intelligible to me。〃 （〃More Letters〃； Vol。 II。 page 373。）
The observations on Linaria and the investigations of the results of legitimate and illegitimate fertilisation in heterostyled plants were apparently the beginning of a long series of experiments。 These were concerned with plants of different families and led to results which are of fundamental importance for a true explanation of sexual reproduction。
The experiments were so arranged that plants were shielded from insect… visits by a net。 Some flowers were then pollinated with their own pollen； others with pollen from another plant of the same species。 The seeds were germinated on moist sand； two seedlings of the same age； one from a cross and the other from a self…fertilised flower； were selected and planted on opposite sides of the same pot。 They grew therefore under identical external conditions； it was thus possible to compare their peculiarities such as height； weight； fruiting capacity； etc。 In other cases the seedlings were placed near to one another in the open and in this way their capacity of resisting unfavourable external conditions was tested。 The experiments were in some cases continued to the tenth generation and the flowers were crossed in different ways。 We see； therefore； that this book also represents an enormous amount of most careful and patient original work。
The general result obtained is that plants produced as the result of cross… fertilisation are superior； in the majority of cases； to those produced as the result of self…fertilisation； in height； resistance to external injurious influences； and in seed…production。
Ipomoea purpurea may be quoted as an example。 If we express the result of cross…fertilisation by 100； we obtain the following numbers for the fertilised plants。
Generation。 Height。 Number of seeds。
1 100 ： 76 100 ： 64 2 100 ： 79 … 3 100 ： 68 100 ： 94 4 100 ： 86 100 ： 94 5 100 ： 75 100 ： 89 6 100 ： 72 … 7 100 ： 81 … 8 100 ： 85 … 9 100 ： 79 100 ： 26 （Number of capsules） 10 100 ： 54 …
Taking the average； the ratio as regards growth is 100：77。 The considerable superiority of the crossed plants is apparent in the first generation and is not increased in the following generations； but there is some fluctuation about the average ratio。 The numbers representing the fertility of crossed and self…fertilised plants are more difficult to compare with accuracy； the superiority of the crossed plants is chiefly explained by the fact that they produce a much larger number of capsules； not because there are on the average more seeds in each capsule。 The ratio of the capsules was； e。g。 in the third generation； 100：38； that of the seeds in the capsules 100：94。 It is also especially noteworthy that in the self…fertilised plants the anthers were smaller and contained a smaller amount of pollen； and in the eighth generation the reduced fertility showed itself in a form which is often found in hybrids； that is the first flowers were sterile。 （Complete sterility was not found in any of the plants investigated by Darwin。 Others appear to be more sensitive； Cluer found Zea Mais 〃almost sterile〃 after three generations of self…fertilisation。 （Cf。 Fruwirth； 〃Die Zuchtung der Landwirtschaftlichen Kulturpflanzen〃； Berlin； 1904； II。 page 6。）
The superiority of crossed individuals is not exhibited in the same way in all plants。 For example in Eschscholzia californica the crossed seedlings do not exceed the self…fertilised in height and vigour； but the crossing considerably increases the plant's capacity for flower…production； and the seedlings from such a mother…plant are more fertile。
The conception implied by the term crossing requires a closer analysis。 As in the majority of plants； a large number of flowers are in bloom at the same time on one and the same plant； it follows that insects visiting the flowers often carry pollen from one flower to another of the same stock。 Has this method； which is spoken of as Geitonogamy； the same influence as crossing with pollen from another plant？ The results of Darwin's experiments with different plants （Ipomoea purpurea； Digitalis purpurea； Mimulus luteus； Pelargonium； Origanum） were not in complete agreement； but on the whole they pointed to the conclusion that Geitonogamy shows no superiority over self…fertilisation （Autogamy）。 （Similarly crossing in the case of flowers of Pelargonium zonale； which belong to plants raised from cuttings from the same 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