EEV. G. HENSLOW — FEETILISATION OF PLANTS. 207 



plant goes on fertilising itself, it cannot introduce new constitutional 

 elements ; but carry it into a new country, and you may see a 

 surprising difference. The water-cress, Avhicli is self-fertilising, 

 now growing in New Zealand, retains the same form that it has 

 here, but it grows twelve feet long, and nearly an inch thick, and 

 has increased so that the Government expends large sums every year 

 in keeping the rivers clear of it. It is not only far superior to, 

 but is completely driving out the native water-plants. 



I will now enumerate several facts which will, I think, make 

 out the case of self- fertilisation. 1. The majority of flowering 

 plants are self-fertile. Mr. Darwin's book brought that out more 

 than I suspected to be the case. The general idea was that con- 

 spicuous flowers are adapted solely for intercrossing. That is not 

 quite true; though a large number of conspicuous flowers are 

 strongly " proterandrous," that is, mature their stamens before their 

 stigmas, and so cannot be self-fertilised ; as the common clove pink ; 

 but the Ipomma purpurea, which is a very conspicuous flower, though 

 freely crossed by bees, yet will fertilise itself if we keep the 

 bees away. 2. Very few plants are known to be physiologically 

 self-sterile. It is a remarkable fact that some plants are in this 

 condition ; but put the pollen on another flower, and it is effective. 

 3. Many are morphologically self-sterile. That means, the pollen 

 cannot reach the stigma of the same flower unless artificially put 

 there, but it is then effective. 4. Self-sterile plants may become 

 self -fertile by many causes. Some of these causes want a little more 

 establishing, it is true; but they have at least been noticed in certain 

 cases. For instance, the withering of the corolla. Mr. Darwin 

 mentions the case of the pansy. I have noticed the same thing in 

 plants in the autumn, when it is getting too cold for the corolla to 

 expand. You may find the corolla withered and pressed down upon 

 the stamens and pistil, which then seeds abundantly. Such a case 

 I have observed in Tradescantia. 5. Loss of colour. That means loss 

 of energy ; because it is known that if you keep balsams without 

 ammonia in the soil, they will be white, but with ammonia they be- 

 come pink again. The conclusion I draw is that from whatever 

 cause the energy in a plant is destroyed, its loss may be indicated 

 by a white or pale-colouied corolla. Loss of energy appears to be 

 favourable for self-fertilisation ; but the converse is not necessarily 

 true. In Mr. Darwin's book there are about half a dozen instances 

 where he mentions that pale flowers and white flowers are more 

 self-fertile than others. 6. The absence of insects. It appears 

 that some flowers which are habitually crossed in their own country 

 by insects, on transportation to another country, where there are 

 none of those insects, may become self-fertilising. Three common 

 examples are the sweet pea, the garden pea, and the dwarf 

 kidney-bean. The pea is really adapted for insect agency, but it is 

 self -fertilising in this country, apparently because we have not the 

 proper insect to fertilise it. If plants therefore do not receive visits 

 from the right insects, the probabiliiy is that they will generally 

 die out, and we have reason to suppose that many have died out 



