840 THE DISPEESION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



where Impatiens, CardaTnine, JDentaria, Oxalis, &c., grow, brush against the fruits 

 of those plants in the course of their wanderings, and at once receive a charge of 

 seeds, some of which are sure to be left sticking to the creature's fur or feathers. 

 It has long been known that when animals pass over places that are overgrown by 

 Materium (see fig. 458^) and brush against its fruits, which hang down from 

 hook-shaped stalks, they are bespattered with the mucilaginous mass in which the 

 expelled seeds are embedded, and that as soon as they reach a place of rest they 

 endeavour to get rid of the unpleasant encumbrance. 



Several contrivances for the distribution of fruits and seeds remain to be 

 described which, so far as regards their results, exhibit the greatest resemblance 

 to the above sling-fruits, although the causes which determine the phenomenon in 

 their case are utterly different. In the last-named the forcible expulsion is due to 

 cellular turgescence, or to movements brought about by the drying up of hygro- 

 scopic cell-layers; in the cases now to be described the result depends solely on the 

 elasticity of stems and fruit-stalks. The stems and stalks in question are strongly 

 resilient, and are strained and curved by a force acting from without. The moment 

 the force ceases to act, their quality of resilience causes them to return to their 

 former position, and in doing so they jerk the fruits and seeds borne by them to a 

 distance. Of these contrivances, which are called balistic means of dispersion of 

 fruits and seeds owing to their analogy to catapults or balistas, we will here deal 

 with five forms. The simplest occurs in the Compositse, whose fruit-capitula are 

 borne upon erect, comparatively long, elastic, flexible stems. The small fruits of 

 the capitulum are already detached from their short pedicels by the time they are 

 ripe and are deposited upon the central disc of the receptacle, which is surrounded 

 by involucral scales, or at the bottom of the basket-shaped fruit-eapitulum into 

 which the floral-capitulum develops. They are so deeply bedded in this situation 

 that it is not possible for them to fall out unless subjected to some external impetus. 

 But the erect resilient stem which bears the capitulum has only to be bent to' one 

 side by a gust of wind or by the touch of an animal for the fruits lying on the 

 fruit-capitulum (which is flat or excavated as the case may be) to be shot off" by the 

 recoil which ensues. In many of the Compositss the involucral scales which form 

 the enveloping basket bend towards one another at the top so as to constitute a 

 roof; they are, however, elastic and flexible and very smooth on the inner surface, 

 so that the fruits when ejected easily slip by them, and yet are to a certain extent 

 guided in the course they take by the tips of the scales. In other Composites, of 

 which the genus Telekia is an example, the floral receptacle is thickly clothed with 

 so-called palese, and the fruits to be ejected, which, it may be noted incidentally, have 

 no pappus, are embedded amongst these palese. The palese are erect and stiff; and 

 are edged with small, upturned teeth; the slightest shock sends the fruits a little 

 higher up amongst the scales, and they cannot then return to their former position, 

 as the stiff- marginal teeth bar the way. The fruits thus seem to make their way 

 up the scales, step by step, as though they were ladders. If, when they have nearly 

 reached the top, there comes a gust of wind which sets the peduncles of the capitula 



