846 THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



of fruit and do not open until the winter rains set in and cause the "rose" to 

 unfold, whereupon the fruits are washed away. 



Similar phenomena in connection with the rainfall are exhibited by the fruits 

 and seeds of the so-called Ice-plants (Mesembryanthemum) which occur in a great 

 variety of forms at the Cape. The capsular fruits of these plants remain closed in 

 dry weather; but the moment they are moistened the valves covering the ventral 

 sutures of the fruit-loculi open back, dehiscence takes place along the ventral 

 sutures, and the seeds, hitherto retained in a double shroud, are washed out of the 

 loculi by the rain (see figs. 463 3>4 ' 6 ' 6 ). Amongst plants belonging to the flora of 

 Europe, the Yellow Stone-crop (Sedum acre) responds to the influence of rain in a 

 manner which reminds one strongly of the Ice-plants above referred to. The carpels 

 are arranged radially, and are furnished at the base with wing-like borders, whilst 

 the central part of the external surface of the fruit is in the form of a shallow basin. 

 In dry weather the five fruit-loculi are closed; but the moment a drop of rain falls 

 upon the concave centre they open wide, and the next few drops wash out the 

 seeds, which are of small size, and convey them to the ground. As the rain trickles 

 into the tiniest crevices in rocks and walls, the seeds are carried into holes in 

 vertical or even overhanging cliffs where it would be scarcely possible for them to 

 be deposited by any other means of dispersion. In the case of Veronica Cymba- 

 laria, which grows on walls in the south of Europe, the fruits likewise remain 

 closed so long as the weather is dry and only open when they are thoroughly 

 soaked. The seeds are then carried, like those of the Stone-crop, into the holes 

 and crevices of vertical walls by means of the infiltration of rain. Similarly in the 

 cases of Veronica agrestis and Veronica serpyllifolia, species of Speedwell which 

 grow profusely on cultivated ground, the seeds are washed out of the gaping 

 capsules by rain and conveyed to spots where they find conditions favourable to 

 germination. It is worthy of note that the capsules of Veronica Anagallis, V. 

 Beccabunga, and V. scutellata, species which grow on banks and in running water, 

 also do not open until they are thoroughly wetted by rain. The explanation of 

 this curious fact must be as follows. If the wind were to act as the means of 

 dispersion there would be a risk of the seeds being deposited on dry places where 

 they would be doomed to perish. On the other hand, the rain carries the seeds on 

 to the wet soil of the marsh or into the shallow water of the brook or pond, as the 

 case may be, where the plant in question finds favourable conditions. 



I must again repeat that actual contrivances with a view to seeds being washed 



out of open fruits by rain are comparatively rare. This, of course, does not exclude 



the possibility of fruits or seeds unprovided with such contrivances being dispersed 



by rain, or by the little tributaries of rivulets, which result from showers of rain if 



nee they are transferred by any means into the channels in question. The rills of 



: which run swiftly down to join larger streams after a violent fall of rain 



b not only sand and earth, but also any seeds that may have been deposited on 



by the wind, and they subsequently set them down with the mud at the 



the stream. Those fruits and seeds also which fall by chance into running 



