sect, ii PHYSIOLOGY 285 



poisons. Thus, for example, it is known of certain Orchids that pollination with 

 their own pollen causes the death of the flower, while in other cases the pollen is 

 killed in a short time by the stigmatic fluid. 



In other instances, self-fertilisation occurs where cross-pollination 

 either is not effected, or else in conjunction with it (Wheat, Barley, 

 Canna, Viola species, Linum usitatissimum, etc.). By many plants, in 

 addition to the large flowers adapted to insect pollination, small, 

 inconspicuous flowers are produced which, usually concealed under- 

 ground or by the lower leaves, never open, and only bear seeds which 

 have been produced by self-fertilisation. In some plants the majority 

 of the seeds are derived from such cleistogamous flowers (Viola), 

 and sometimes their seeds alone are fruitful (Polycarpum tetraphyllum 

 possesses only cleistogamous flowers). As the greater number of such 

 plants, however, in addition to the seeds of the self-fertilised small 

 cleistogamous flowers, produce others resulting from the cross -fertilisa- 

 tion effected in the larger flowers (Impatiens noli-tangere, Lamium 

 amplexicaule, Specularia perfoliata, etc.), the ancestral plants of the 

 cleistogamous generations, as well as their descendants, have, at least, 

 the opportunity for cross-fertilisation open' to them. 



Special contrivances for assuring the crossing of the sexual cells, 

 particularly by preventing self-pollination, are found to exist 

 throughout the whole vegetable kingdom. 



Self-pollination is most effectually avoided when the plants are 

 unisexual, that is when both male and female plants lead a separate 

 existence. Such DIOECIOUS plants exist in almost all classes of plants 

 from the lower Cryptogams to the most highly developed Phanerogams 

 (many of the lower Algae, species of Fucus, Marchantia, Polytrichwm, 

 Equisetaceae, Taxus, Hemp, Hops, Date-Palm, etc.). In moncecious 

 plants the male and female organs occur on different flowers, but 

 the flowers are borne on the same plants. The fertilisation between 

 different flowers is thus secured ; but even here crossing with other 

 individuals is, for the most part, assured by dichogamy. 



The term dichogamy is used to denote the fact that the male and female 

 sexual organs attain their maturity at different times. When either the male or 

 female sexual organ matures before the other, the self-pollination of morpho- 

 logically hermaphrodite flowers is avoided and crossing assured. Both herma- 

 phrodism and moncecism are more advantageous than dicecism, as all the 

 plants in such cases are able to produce seeds ; while in dioecious plants the male 

 flowers cannot be utilised for the direct production of seeds. Dichogamy secures 

 crossing in such a simple manner, and is so easily attained by hermaphrodite 

 plants, that it is of very general occurrence in the vegetable kingdom. According 

 to the priority of the maturity of their sexual organs, plants are designated 

 PROTANDllOUS or PEOTOGYMOUS. 



Pkotandry, the earlier maturing of the male sexual organs, is the more frequent 

 form of dichogamy. It occurs in the flowers of the Geraniaceae, Oampanulaceae, 

 Compositae, Lobeliaceae, Umbelliferae, and inEpilobium, Digitalis, etc. of the Mal- 

 vaceae. The anthers, in this case, open and discharge their pollen at a time when the 



