170 



KNOWLEDGE, 



[AufiusT 1, 1896. 



Fio. 2.— 



Fern Spore- 

 Case, showing 

 Annulus (en- 

 larged). 



support suspended by a fibre of catgut, wbicli twists and 

 untwists according to tbe state of tbe atmospbere. The 

 instrununt is really a hyKrofcope or hygrometer indi- 

 cating the humidity of the air, and the figures are made to 

 swing round by hygroscopic action, 'i'he proportion of 

 moisture present in the atmosphere can bo ascertained 

 with far greater precision, however, by means of tbe wet- 

 bull) hydrometer or by Danieli's dewp.iiiit liygrometer. 



The dehiscence of seed capsules is brought about by tbe 

 contraction of their outer walls, and this often takes place 

 in such a way that the seeds are violently 

 expelled, as in the ripe pods of the brooin, 

 vetch, and bird's-foot trefoil. At the 

 moment of dehiscence each half of the 

 legume or pod suddenly twists itself, scatter- 

 ing the seeds with great force. The fruit of 

 the common dog-violet is of interest ; each 

 of its three valves, when the fruit opens, has 

 a double row of seeds. The valves are boat- 

 shaped ; in drying they contract till the 

 seeds are so closely jammed in and the 

 tension becomes so great that the slightest 

 touch causes them to be shot up into the air. 

 The seed vessel of Veronica, on the other 

 hand, remains closed in dry weather, but 

 opens and discharges its seeds in wet. The 

 application of moisture also causes the 

 stellate capsule of Mesembryanthemum to 

 open up. Instead of opening to allow of the escape of 

 its seeds, a many-seeded fruit may become broken 

 up into indebiscent pieces, each containing a single 

 seed. The fruit of the dead-nettle, called a carcerulus, 

 splits up in this way into four little nutlets. In some 

 of the Labiatffi, such as self-heal, FruneJla, the nutlets 

 are retained within the calyx, which remains closed 

 while the weather is fine ; rain, however, causes the sepals 

 to open up and the nutlets are discharged. The fruits of 

 the Geraniaceff, or cranes'-bills, are also schizocarps, as 

 these splitting fruits are called. These plants are named 

 from tbe long beak or carpophore of their peculiar fruit. 

 To this beak the five one-seeded carpels at first adhere, but 

 when ripe their basal portions separate from it and curve 

 upwards. At tbe same moment each carpel splits open 

 along its inner edge, and by tbe suddenness of tbe move- 

 ment the seed is projected out to a distance of some feet. 

 A few species have indebiscent carpels, which themselves 

 spring away from tbe carpophore. This is what happens 

 in the stork's-bill, Krodium, a rather common plant 

 belonging to tbe geranium family. The 

 fruit of Krodium is, perhaps, tbe most 

 interesting of all the hygroscopic class. 

 ^^'hen it springs away from the carpophore 

 in tbe manner just described, the carpel 

 of Krodium is seen to have a long slender 

 filament or awn, which is in the act of 

 curling upon itself. After the carpel 

 alights its awn continues to curve and 

 twist for a minute or two, until at last it 

 acquires a corkscrew form and comes to 

 rest. As long as tbe awn is kept dry it 

 does not change, but if moistened with a 

 drop of water it immediately begins to 

 straighten out, and in the course of a very 

 few miuutes becomes quite straight. Two 

 or three long slender hairs near the base 

 of the awn keep tbe carpel in position, with its sharp point 

 directed obliquely into the soil. If now, as in all pro- 

 bability is the case, the upper extrercity of the awn presses 

 against some object which affords a point of resistance, the 



Fig. 3.— Le- 

 gume of Bird's- 

 toot Trefoil alter 

 dehiscence, show- 

 ing the twisted 

 TaiTes. 



Fro. 4. -Fruit 

 of Stork'8 ■ Bill 

 (A't'odiinn rirtf- 

 lariiim), the Car- 

 pels still attaclied 

 to the Cavpo- 

 phoro. 



moistening, in consequence of the untwisting and elonga- 

 tion of the awn, forces the point of the carpel down into 

 tbe earth. Should the soil after a time get dry, the awn 

 will once more assume its corki-crew condiiinn ; but instead 

 of the seed being drawn up by this sliortpning of the awn, 

 the latter is itself drawn down, for the seed holds on and 

 keeps all it gains, its point being barbed like a harpoon. The 

 chances are now that the upper extremity of the awn has 

 been applied to a new point of resistance, 

 so that the next rain will send the seed 

 still deeper into the soil, and so on with 

 every succeeding change of weather. The 

 awn in some species is feathered at its 

 extremity, the more certainly to secure 

 that it shall press against a resisting 

 object when it begins to unwind. Species 

 of anemone and clematis have awncd 

 seeds or achenes capable of burjing them- 

 selves in this manner. Tbe feather-grass 

 (Stipii pennatii) has the same peculiarity ; 

 we have found its seeds stuck deep in the 

 ground like so many darts. Seeds of this 

 description , several of which have been used 

 in the construction of hygroscopes, may 

 literally be said to screw themselves into 

 the earth. The grains of the barren oat and some other 

 grasses have hygroscopic awns by means of which they 

 move about from place to place. During damp weather a 

 heap of these grains disperse themselves in a most sur- 

 prising manner, marching off in various directions like so 

 many flies. In other cases, as in the common vernal grass 

 and one or two others, the seed trips about in quite 

 fantastic style. Many aristate grains and seeds appear, 

 however, to be adapted for dispersion through animal 

 agency. Even in such cases the hygroscopic property is 

 useful ; it enables tbe seed to burrow into fur or wool the 

 better to ensure its transport. A 

 remarkable splitting fruit is that 

 of llurti crepitans, called the 

 " monkey's dinner-bell" because 

 its woody carpels when quite dry 

 separate suddenly with a noise 

 like the report of a musket. The 

 common wood-sorrel has the 

 testa or outer coat of its seed 

 elastic ; if touched, this suddenly 

 ruptures, and the core of the seed 

 is projected to some distance. 

 The hairy cress [Cunhiminc 

 liimuta) is a well-known example 

 of an elastic fruit. If tbe ripe 

 siliqua be touched, its two valves quickly curl up and the 

 seeds are shot out all around. The pod of Impatiens, or 

 " touch-me-not," behaves in the same manner. 



A sharp line can hardly be drawn between hygroscopic 

 movements and those due to turgescence. Of the latter 

 the squirting cucumber furnishes an illustration. When 

 the fruit breaks away from its stalk, the seeds, along with 

 tbe watery contents, are shot out as from a syringe. 

 Turgescence aids in tbe dispersion of a number of fungi. 

 The mortar-fungus {S/ih/ernliolus) projects its sporangium 

 very much in the way a shell is thrown from a mortar. 

 The globular sporangium retained by the outer membrane 

 of the peridium is pressed from behind, till tbe tension 

 suddenly ruptures the membrane and the sporangium is 

 forcibly expelled. The rose of .Jericho, a small annual 

 found in sandy places in Syria, has the curious habit of 

 curling itself up into a ball ; the rootlets become detached 

 from the dry earth, and the plant is rolled along the 



Fio. .5. — Carpel of Erodium. 

 1. Ury condition. 2. After 

 moistening. 



