844 



THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



take place provided the part of the awn which undergoes it is not iixed down 

 anywhere by an obstacle. If one of the lever-arms of the awn encounters a fixed 

 object on the ground the spiral motion of the lower arm sometimes has the effect 

 of forcing the tip of the awn over the obstacle, so that the whole structure shoots 

 obliquely upwards. This phenomenon is especially conspicuous in Avena sterilis. 

 In this case two glumes furnished with strong bent awns are to be seen on the 

 fruiting spikelet after it has fallen. An alteration in the environment in respect 

 of moisture causes the two awns to twist in opposite directions, so as to cross one 

 another. After pressing one upon the other, they ultimately slip apart with a 

 sudden jerk, which causes the whole fruit to spring up. This movement is much 

 more like hopping or jumping than creeping. ^ 



The distance traversed by creeping, hopping, and bounding fruits is seldom 

 greater than a few decimetres. The movements generally land the fruits almost 



4 Calyx-tooth of 



Fig. 462. — Fruits which creep or hop along the ground. 



' Awn of ^gilops ventricosa. 2 Awn of ^gilops ovata. 3 Bristles of the pappus of Crupina 

 Trifolium stetlatum ; all the figures magnified. 



immediately in some cul de sac, where they remain, or else the awns gets entangled 

 with the above-ground stems and leaves, and in that case the result of the move- 

 ments is to imbed the seeds, which are concealed in the fruit-scales, in the earth 

 (see vol. i. p. 617). In such cases no doubt the most important function of the 

 movements in question is to fix the plants in the soil, but on the other hand it 

 cannot be denied that a limited form of dispersion may be and is as a fact achieved 

 by these movements. 



The dispersion of fruits through the agency of water takes place in all plants 

 which undergo fertilization under water and detach their fruits when they are 

 quite ripe. To this class of fruits belong the Fungi of the family Saprolegni- 

 acecB, and most of those Cryptogams which are known as Algse. Such facts as 

 have been ascertained by botanists concerning the distribution of the fruits of these 

 plants in the water have been recorded in previous pages (see pp. 49 and 64). This 

 method of dispersion is of less importance in the case of Phanerogams, which are 

 fertilized and ripen their fruits in the medium of the air. At first sight one might 



1 The hopping movements of the fruita of the Mexican plant named Seiastiana Pavoniana, and of those of 

 Tamarix Galiica, which belongs to the Mediterranean floral area, are not due to alterations in the tension of 

 particular parts of the fniit-coat, but are caused by insect-larvae which live inside the fruits. In the case of the 

 " Mexican Jumping Bean ", the larvse are those of the small lepidopterous insect named Carpocupsa saltitans, and 

 in Tamarix Oallica those of the beetle J^aiiodes Tamarisci. 



