i28o A TEXTBOOK OF THEORETICAL BOTANY 



cross-pollination. Later investigations have shown that heterostylous 

 plants are no exception, but autogamy seems to be confined to one only of 

 the flower-types produced by each species. Where this is the short-styled 

 form, the stigma receives pollen falling directly from the anthers above it, 

 provided that the position of the flower is vertical. Where the flower is not 

 vertical, it is usually the long-styled flower which is autogamous. The 

 corolla may dehisce as a unit and in slipping over the style bring the short 

 stamens up to the stigma, which is thus pollinated by the remaining pollen. 

 \'arious devices of bending or grow^th of the floral parts may also operate 

 in heterostylous as in other types of flower to bring about self-pollination. 



It has been pointed out that in dichogamous flowers the first protogy- 

 nous flowers and the last protandrous flowers cannot be pollinated by their 

 own species and hence are amenable to hybridization. The more complete 

 dichogamy is, the greater are the chances of foreign pollination, which may 

 sometimes be effective. Similarly the pollen of the first flowers in protan- 

 drous species cannot be used to pollinate flowers of their own species, since 

 no stigmas are yet receptive. Variation in the flowering times of different 

 plants of the species may alleviate this difliculty but cannot altogether over- 

 come it and an ineffective phase must exist, if only a short one. Hence the 

 importance of incomplete dichogamy, which allows an overlapping period 

 when both sexes are active. It is much commoner than complete dichogamy. 

 Indeed if a dioecious species, such for example as a Willow, were completely 

 dichogamous it could hardly continue to exist. Even the incomplete 

 dichogamy of Salix allows for a high frequency of hybrid pollination. The 

 Dutch Elm, Ulmus hoUandica, var. belgica, shows an extreme case of this 

 difliculty. The flowers are hermaphrodite but highly protandrous and all 

 the flowers shed their pollen at the same time. As none of the embryo-sacs 

 are then ready for fertilization a large proportion of the pollen is wasted and 

 most of the pollination is from other species of Elm, with a high degree of 

 sterility in consequence. 



We have seen above that illegitimate pollinations in heterostylous flowers 

 are usually ineffective. This is only a special case of the general pheno- 

 menon of self-sterility or its analogue, incompatibility. Self-sterility 

 is known to exist in many plants and probably is widespread. Among 

 cultivated plants, especially in fruit trees belonging to Rosaceae, it may be of 

 great economic importance. It is not, as a rule, a uniform feature through- 

 out a whole species, but may vary from population to population or from 

 individual to individual. It may coexist with regular self-pollination, 

 which, of course, it renders useless, and it is a complete barrier to autogamy. 

 The allied condition of incompatibility is one in which the flower, generally 

 self-sterile, is also sterile to pollination by certain other strains of its own 

 species. We are not simply dealing, in such cases, with two sexes, but with 

 a number of specialized strains in each sex, among which sexual unions are 

 limited to certain strains only. There is more than a hint here of analogy 

 with the condition of multiple heterothallism which obtains among the 

 higher Fungi. Heterothallism itself indeed is, basically, self-sterility^ 



