1658 A TEXTBOOK OF THEORETICAL BOTANY 



open the stamens diverge so widely that even short-tongued insects can 

 thrust their heads between the filaments and the carpels to obtain the 

 nectar, which is secreted by a ring at the base of the flower. The outermost 

 anthers dehisce first and turn their valves outwards while the stigmas 

 mature at the same time. At this stage, therefore, most insect visitors 

 perform cross-pollination, and the flower is pollinated before all the anthers 

 are ripe. Automatic self-pollination very rarely occurs and then only 

 towards the end, when the innermost anthers dehisce. Because of the 

 amount of nectar secreted insect visitors are very numerous and self- 

 pollination normally only occurs in very bad weather. 



In contrast to these examples we have the species Poterium sangiiisorba 

 in which the flowers are nectarless. The inflorescence is a close spike, 

 composed of female flowers at the top, followed below by hermaphrodite 

 ones and finally at the base by male flowers. The female flowxrs have 

 conspicuous red, bushy stigmas. The hermaphrodite flowers are usually 

 homogamous but this varies in individual plants. The anthers are yellow 

 and pendulous with long reddish filaments and in most inflorescences the 

 male flowers greatly outnumber the female, while hermaphrodite flowers 

 may be scattered among the unisexual ones. Hover flies occasionally visit 

 the flowers in a profitless search for nectar, but, on the whole, the plants 

 rely on the wind for pollination. Although mechanisms to ensure cross- 

 pollination are so marked a feature of the family, it should be noted that in 

 certain genera, particularly Rosa, Riibiis, Crataegus and Alchemilla, there is 

 very frequent apogamy, many embryos being developed without fertiliza- 

 tion, so that they carry only the maternal characters. This peculiarity, 

 continued with frequent inter-specific crossing, has resulted in the production 

 in these genera of large numbers of very closely related " species " whose 

 taxonomic status is often doubtful. More will be said about this subject in 

 Volume HI. 



Another interesting point bearing on cross-pollination is the prevalence 

 of self-sterility, especially in the genera Pyrus, Primus and Fragaria. This 

 affects some of the most important cultivated fruits, and must be taken 

 seriously into account by the cultivator who finds that a self-sterile variety 

 grown by itself will yield no fruit and must be cross-pollinated by the 

 introduction of other suitable varieties as pollinators. 



Before bringing our account of the Rosaceae to a close we must refer 

 briefly to certain species of economic importance whose fruits, in warmer 

 climates, take the place of the temperate Apples, Pears and Cherries. 



The Prune [Prunus domestica var. Juliana) is cultivated mainly in the 

 warmer parts of Europe and particularly in the Mediterranean region. It 

 is a variety of the wild Plum. Those grown in France are dried in large 

 quantities and form an important commodity for export. The Balkans also 

 at one time exported prunes. Luther Burbank, experimenting on breeding 

 new varieties of plants, produced a stoneless Prune in which the seed was 

 replaced by a jelly. This form does not appear to have been economically 

 exploited. More typically subtropical is the Loquat {Eriobotrya Japonica), 



