406 FERTILIZATION AND FORMATION OF FRUIT IN PHANEROGAMS. 



transferred thither a good crop of seed results. Artificially produced autogamy in 

 hermaphrodite flowers, in which the stamens are all of one length, is generally 

 productive, nor does it appear to matter whether the pollen used for pollination be 

 u from the first stamen to open or the last. The number of species in which 

 artificial autogamy is unfruitful is extremely small. Crambe tataria, Draba repens, 

 Liliuiii hulhlferum, Lysimackia nummularia, and a few Orchids and Papilionaceae 

 may serve as examples, though even in these cases it is quite possible that some 

 source of error, such as was mentioned at the commencement of this chapter, has 

 been overlooked. 



We may now proceed to discuss what is known as the prepotency of foreign 

 pollen over own pollen. The term " foreign " is used of pollen upon a stigma which 

 lias been brought from another flower of the same or of some other species; "own" 

 pollen, on the other hand, is applied to such as has originated in one of the anthers 

 of the same flower. These terms are employed for the sake of brevity. If one 

 examines a flower of Corydalis early in the morning of the day on which that fli iwer 

 will become accessible to insects, one finds that the anthers have already dehisced, 

 and that the stigma is covered with own pollen. The stigma, lying between the two 

 spoon-shaped petals, is regularly embedded in pollen. But as yet the stigma is 

 immature and unreceptive, so that the absence of any interaction between poll a 

 and stigma at this stage is intelligible. When insects come in due course, a port i< m 

 of this pollen will be removed (cf. p. 266). Should the insects have visited Corydalis- 

 flowers previously, they will leave some of the foreign pollen with which they are 

 dusted upon the stigma at the moment when they remove some of the own pollen. 

 The stigma is now in contact with both own and foreign pollen, nor will additional 

 insect-visits materially alter this state of affairs. In due time the stigma becomes 

 receptive and exerts a selective action upon the pollen. Though the process, as it 

 takes place here, cannot be followed step by step, still we are justified in assuming, 

 on the results of many experiments of artificial pollination, that the foreign pollen 

 receives the preference. It has been shown for Corydalis cava that the flowers are 

 absolutely barren to their own pollen, only slightly fertile to pollen from another 

 flower on the same plant, and only thoroughly fertile when impregnated with pollen 

 from a different plant. For other species, however, e.g. Corydalis cap n aides, fabacea, 

 and ochrolevxa, it has been shown that the plants are fertile to their own pollen, so 

 that if no insect- visitors come, the flowers do not remain sterile. 



These results show how fallacious it would be to make the condition obtaining 

 in Corydalis cava the basis of any far-reaching generalization, such as that auto- 

 gamy is prevented, and without result. In point of fact, autogamy is highly pro- 

 ductive in most species of Corydalis, and occurs, in such plants as are unvisited by 

 insects, in the closed flower in a manner recalling that form of autogamy known as 

 ckistogamy {cf. p. 391). That foreign pollen is prepotent in Corydalis co/mold':-: 

 fabacea, ochroleuca, &c, when both foreign and own pollen are present on the stigma 

 together, is neither asserted nor denied, though, in view of all the circumstances, it 

 seems probable. 



