560 



BOTANY 



PART II 



The effect of HETEROSTYLY discovered by DARWIN is similar. 

 According to TlSCHLER this condition can be altered by the conditions 

 of nutrition. A good example is afforded by Primula sinensis (Fig. 

 533). Comparison of the flowers on different individuals shows that 

 they differ as regards the position of the stamens and stigma. There 

 are long-styled flowers, the stigma standing at the entrance to the 

 corolla-tube, while the anthers are placed deep down in the tube ; and 

 short-styled flowers, the stigma of which stands at the height of the 

 anthers, and the stamens at the height of the stigma of the long-styled 

 flower. An insect will naturally only touch organs of corresponding 

 height with the same part of its body and thus carry pollen between 

 the male and female organs of corresponding height. Thus cross- 

 pollination is ensured. The relative sizes of the pollen grains and 

 stigmatic papillae agree with this cross-pollination. 



FIG. 533. Primula sinensis: two heterostyled flowers from different plants. L, Long-styled, 

 K, short-styled flowers ; G, style ; S, anthers ; P, pollen-grains, and N, stigmatic papillae of 

 the long-styled form ; p and n, pollen-grains and stigmatic papillae of the short-styled form. 

 (P, N, p,n,x 110. After NOLL.) 



The same DIMORPHIC HETEROSTYLY is exhibited by Pulmonaria, 

 Hottonia, Fagopyrum, Linum, and Menyanthes. There are also flowers 

 with TRIMORPHIC HETEROSTYLY (LyiJirum salicaria, and some species of 

 Oxalis) in which there are two circles of stamens and three variations 

 in the height of the stigmas and anthers. 



In a great number of flowers self-pollination is made mechanically 

 impossible, as their own pollen is prevented by the respective positions 

 of the sexual organs from coming in contact with the stigma 

 (HERCOGAMY). In the Iris, for example, the anthers are sheltered 

 under the branched petaloid style. The pollinia of Orchis are retained 

 in position above the stigma ; in Asdepias the five pollinia are attached 

 in pairs to swellings of the style by adhesive discs (cf. Fig. 746). 



Sometimes hercogamy and dichogamy occur together. The flowers of 

 Aristolochia clematitis (Fig. 534) are protogynous. The conveyance of pollen 



