THE ANGIOSPERMAE 1351 



What will be our general impression if we survey what we know of 

 pollination in different families, arranged in systematic order? If there is 

 any value in our classifications they would give us at least an inkling of the 

 drift of evolution. What has been the trend in the flower? Certainly an 

 increasing economy of means may be deduced, coupled in many cases at 

 least with increased sexual efficiency. If we postulate an ideal condition of 

 sexual equality, with pollen grains and ovules equal in number and no wast- 

 age, then obviously it has nowhere been attained. Even in the Orchidaceae 

 there are at least twenty pollen grains for every ovule. This apparent super- 

 fluity may, however, be a source of genetic advantage, since germinal selec- 

 tion is a very real force and it might well be a source of weakness to the 

 species that the genome of all pollen grains alike should be perpetuated. 



Alongside, however, of this trend towards increased efficiency, which 

 is sufficiently obvious when extremes are contrasted, there goes another 

 tendency, towards decreased fertility, shown by reduction in the numbers 

 of sexual parts and more especially in the number of ovules. Here the 

 Orchids are an outstanding exception, but putting them aside, the tendency 

 towards uniovulate or pauciovulate carpels in the higher families is well 

 marked. A balance may, of course, be maintained by the production of 

 greater numbers of small flowers, but at how much greater an expense of 

 material and with what a multiplication of the chances of failure. Sexual 

 failure, indeed, is of widespread occurrence, even, once more, among the 

 Orchids, with their wonderful pollination mechanisms. Its menace hangs 

 over all the floral world and many species and even whole genera appear 

 to have evaded the issue by relying exclusively, or almost exclusively, on 

 self-pollination, with whatever genetic disadvantage it may entail. Other 

 species which we have noted earlier (p. 1281) have gone further and have 

 almost abandoned seed propagation in favour of vegetative propagation. 

 We must not read too much into such observations, but they do carry a dis- 

 turbing suggestion of a secular drift towards decreasing fertility, if not 

 sterility. 



One of the most remarkable ways in which the risks of pollination are 

 avoided is the phenomenon of cleistogamy. By this is meant the produc- 

 tion of flowers which never open and in which self-pollination is carried out 

 within the closed bud, which then passes directly into fruit. 



The list of genera in which cleistogenes, as these flowers are called, have 

 been observed is a long one. Darwin named 67 genera, though unquestion- 

 ably the list is far from complete. It includes the following: Viola, Oxalis, 

 Lamiiim, Ajiiga, Salvia, Saxifraga, ScrophuJaria, Streptocarpus, Lycopus, 

 Drosera, Subularia, Jimciis, Comnielina, Lathraea, Stellaria, Cardamine, 

 Montia and Ononis, to mention only the well-known genera. Further, it is 

 widespread among grasses. Chase has recorded cleistogamous flowers in 

 twenty grasses in the United States, including all the native species of 

 Triplasis, Danthonia and Cottea and in Muehlenbergia microsperma and 

 Pappophorum urightii. Four South American species of Danthonia and one 

 irom New Zealand also produce them, as do Stipa pennata and Stipa leiico- 



