«42 THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



The manner in which the fruits of Labiatse are thrown off is particularly 

 remarkable. The fruits in question are spherical, oval, or ellipsoidal nutlets, and 

 when ripe are still hidden at the bottom of the persistent calyx. The calyx is 

 either bell-shaped or tubular, and faces laterally; the pedicel supporting it is 

 resilient, and usually bowed (see figs. 460 ^' ^> ^' ^). If one presses upon the stiff 

 points of the calyx with some hard object such as a piece of wood (fig. 460 ^) the 

 pedicel is subjected to a strain, and as soon as the pressure ceases it springs back to 

 its former position, and the nutlets are shot out with great force (fig. 460^). The 

 path of projection is in this case determined by the two inferior calyx -teeth, 

 which curve upwards like sledge-runners (see fig. 460 ^). In many instances, as, 

 for example, in Teucriitm flavum, T. Euganceum, and Monarda fistulosa (see 

 figs. 460 ^' ^' ^' ^' ^' ^' ^), there is yet another contrivance for ensuring the proper 

 direction of flight. This consists in the presence of stiff though pliable convergent 

 hairs in the calyx-tube, and their function may be compared to that of the grooves 

 in a rifle. Again, in Scutellaria the lobes of the calyx-limb, which is in the form 

 of a tilting helmet with the visor down, determine the path of the seeds after 

 expulsion. The result thus artificially attained by bending down the stalks of the 

 fruiting calyces and letting them fly up again is brought about in nature by gusts 

 of wind, by drops of rain, and most frequently of all by animals brushing against 

 the calyces. In the last-mentioned event one or other of the ejected nutlets may 

 stick to the animal's coat and be carried to a much greater distance than would 

 otherwise be the case. This kind of apparatus rarely occurs in plants other than 

 Labiatse. The nearest analogy is found in the ejection of seeds from the fruits of 

 several species of the Chickweed genus, e.g. Cerastium macrocarpum (see p. 448, 

 fig. 340 *), where the fruits are curved like the letter S, are borne on stiff stalks and 

 hold the ends that open upwards. 



One of the most curious forms of mechanism of the catapult variety occurs in 

 the North American Polygonum Virginicum (see figs. 460 ^^ and 460 ^^). In this 

 plant the fruits are on short stalks, and are arranged in spikes on long switch-like 

 stems. The fruit-stalks are remarkable for the fact that the cells of the cortical 

 parenchyma, which is greatly developed, have their walls strongly lignified, though 

 only slightly thickened. It is also noteworthy that between the stalk and the fruit 

 there is a layer of separation which looks like a joint to the naked eye. The style 

 is transformed into a decurved beak, which is seated upon the fruit, and terminates 

 in two little divergent hooks. When one of these fruits is pushed by a passing 

 animal it is at once detached at the separation- layer and springs away to a distance. 

 The pressure applied to the fruit is apparently transmitted to the short stalk, and 

 gives rise to a condition of tension in the tissue of the stalk analogous to that of a 

 watch-spring. As soon as the pressure ceases the tension relaxes, and the fruit is 

 cast away with great force. For a long time it remained a mystery how these 

 fruits were thrown off" in the absence of any animals to give the initial impulse. 

 A few years ago, however, I succeeded in observing the manner in which the long 

 fruiting switches are swayed backwards and forwards by a boisterous wind. 



