THE LIFE-mSTOEY OF PLANTS. 



189 



tlie result of close fertilisation. Every provision is 

 liere made for close fertilisation, little or none for 

 cross-fertilisation. These flowers are called " cleisto- 

 gamic " (from the Greek kleio, "I close, or shut"). 

 In the Yiolet then, as indeed in many other plants, 

 there are some fl.owers adapted for close, othera for 

 cross fertilisation. Close fertilisation would neces- 

 sarily tend to the perpetuation by inheritance of the 

 characters of the plant. Cross-fertilisation, on the 

 other hand, seciu-es the possibility of variation in the 

 offspring, and therefore a greater power of adapta- 

 tion to different circumstances. In the long run, 

 too, as has been shown by Darwin, and as indeed is 

 recognised by all breeders of plants, or of animals, 

 close, or "in-and-in," breeding is objectionable as 

 increasing the chances of debility in the offspring, 

 and facilitating the onset and progress of disease. 

 These injurious effects are averted and obviated by 

 at least occasional cross-fertilisation. 



In the dimorphic flower just mentioned the shape 

 of the two flowers is so different as at once to attract 

 attention, but the same results are brought about in 

 some cases with much less striking changes of ap- 

 pearance. 



Pin -eyed and Thrum-eyed Flowers. — 



Gardeners for ages have noticed a difference in the 

 flowers of the Polyanthus and of the Auricula. In 

 some the orifice of the flower-tube is seen to be filltd 

 up with a little greenish knob like the head of a pin, 

 and such flowers were accordingly called " pin-eyed ; 

 in others, the " eye " of the flower, or top of the 

 corolla-tube, is filled with a conical yellow mass 

 resembling thrums or thi-eads, and hence the name 

 " thrum-eyed." But, although these different ap- 

 pearances had been familiar for ages, and not only 

 familiar, but made the objects of special attention 

 in carrj-ing out the cainice of the "florist," yet it 

 was left for Darwin to bring out the true significance 

 and purport of these differences of arrangement by 

 means of direct comparative experiment. In fact, 

 the ''pin-eye" above-mentioned is nothing but the 

 stigma of the Primrose, the "thrum" is nothing 

 but the group of anthers. In the one flower the 

 stigma, in the other the anthers, occupy the mouth 

 of the corolla-tube, or protrude from it. On slitting 

 down the corolla-tube so as to see its interior, it will 

 be found that in the pin-ej^ed flowers the style is 

 long, and so the stigma, or pin, is raised to occupy the 

 mouth of the corolla, while the stamens are short, 

 and low down in the tube. In the thrum- eyed 

 flowers, on the contrary, the stamens are long, and 

 the style is short. By actual experiment and observa- 

 tion Dar-s^-in proved that the most perfect degree of 

 fertility — that is, the largest number and healthiest 

 seedlings — ensued when cross-fertilisation took place 



between the pollen from the short stamen (thrum- 

 eye) and the long style (pin-eye), and inversely. 

 This may be thus represented : — 



Thrum-eyed flower. Pin-eyed flower. 



r Anthers protruding-.^^^^ ^^Anthers included ■~\ 



C Stigma includeC--^ ^^Stigma protruding) 



From this diagram it may be inferred that cross- 

 fertilisation by insect agency takes place as shown 

 by the direction of the cross-lines, while close 

 fertilisation may take place in the direction of the 

 brackets. 



"We say may take place, to indicate its possible 

 occurrence ; that it does not always happen is shown 

 by the circumstance before mentioned, that pollen 

 and stigma are not always ripe at the same time. 

 Indeed, generally there is a difference in this respect, 

 the anthers of some flowers being expanded first (as 

 in "protandrous flowers"), the stigma of others 

 anticipating the faU of the pollen from its own 

 flower (" protogynous flow^ers "). 



Close fertilisation is also prevented in those cases 

 — of which several are recorded — in which the pollen 

 of a flower is absolutely inert on the stigma of the 

 same flower, though the conditions may be favour- 

 able, and although the same poUen transferred to 

 another flower may impregnate the ovules. This has 

 been observed by both scientific experimenters and 

 by gardeners in the case of Passion-flowers. 



While the general rule is as we have endeavoured 

 to explain it, it must not be supposed that it is 

 invariable. On the contrary, a flower that is 

 habitually cross-fertilised may become close fertiHsed, 

 a flower which is usually protandrous may be- 

 come protogynous, or both stamens and stigma 

 may be ripe at the same moment. Hence, there 

 is a possibility in any given flower of much varia- 

 tion in the method of fertilisation — and the plan 

 followed in one flower of the same plant is not 

 necessarily the same as that which obtains in an- 

 other. Such facts as these fiunish one explanation 

 of the cause of the variations that we see in plants, 

 whether wild or cultivated, and it is ob\aous that 

 they require to be very carefully considered by the 

 hybridiser, or raiser of seedlings. If he require to 

 secure a "fixed strain" or a "pure stock," he will 

 or should study the habit and peculiarities of the 

 flowers, and take every pains to secui-e close and to 

 prevent cross-fertilisation. If, on the other hand, 

 his desire be to raise new varieties, then, guided by 

 the same facts, he will practise cross-fertilisation. 



Protective Adaptations. — The mechanism to 

 secure cross-fertiHsation or close fertilisation, as the 

 case may be, by insect agency or otherwise, is, as we 



