404 THE BIOLOGY OF FLOWERING PLANTS 



remains small, is pushed into the soil. Only there does 

 it begin to grow and mature. How so remarkable a habit 

 can have arisen is difficult to understand. The fruit seems 

 to draw both water and salts from the soil, and can ripen 

 only when buried. This can hardly have been the primary 

 meaning of the process. It is possible that, to begin with, 

 a certain protection from preying animals was achieved, 

 and that subsequently the fruit became more dependent 

 on the soil covering. 



In Trifolium suhterraneum a post-floral movement of the 

 peduncle leads to the burying of the fruiting head, and this 

 is the case with Voandzeia suhterranea and Cyclamen euro- 

 pceutn. In the last the burying of the fruit takes place by a 

 spiral inrolling of the peduncle, in the others by a positive 

 geotropic curvature. After pollination the peduncle of 

 Vallisneria spiralis coils up and draws the ovary to the 

 bottom of the water, where the fruit ripens. 



In Linaria Cymhalaria, the ivy-leaved toad-flax, the 

 flowering peduncle is positively phototropic ; after pollina- 

 tion the peduncle becomes negatively phototropic. On old 

 walls we may frequently see the flowering peduncle carrying 

 the flowers outwards, clear of the leaves, while the negative 

 reaction after pollination brings the fruit into cracks of the 

 wall where the seeds are shed. Here no actual burying 

 takes place, and we have a case where the advantage secured 

 by liberating the seeds in a place suitable for the growth 

 of the plant is clear. The use of many other similar move- 

 ments cannot be said to be understood. In Tropceolum 

 majus, the garden nasturtium, the flowering peduncle is 

 nearly erect and points forward from the foliage. A few 

 hours after pollination a downward curvature takes place 

 in the peduncle just below the ovary, and within 24 hours a 

 second, very sharp, downward curvature takes place about 

 2 in. lower. The result is that the fruit is buried in 

 the foliage. This movement has been analysed by Oehlkers 

 (1921), who finds that it is chiefly due to a reversal in 

 the geotropic reaction, taking place about the time of 

 fertilisation, but independent of that process. In un- 



