620 



COTYLEDONS. 



of the pointer- like end of the Stork's -bill fruit in very damp and in very dry 

 weather respectively, and we can then draw conclusions from the position of the 

 pointer as to the relative dampness of the air. In this application of the fruit we 

 have an exhibition of the torsions which take place in the course of its penetration 

 into the ground, and which are produced by the alterations in the humidity of the 

 air. When such a fruit falls on the ground, however, it is not the lower thick end 

 inclosing the seed which is fixed, as in the hygrometer, but the pointer-like process, 

 and consequently in nature it is the seed-end and not the pointer which is set in 

 motion by an alteration of humidity in the air. The fixing of the fruit in the 

 ground is naturally effected thus: the point of the arm lies on the soil, and in 

 consequence of the untwisting of the spirals in damp weather, the thicker seed- 

 containing end (which is provided with a sharp point) bores deeply into the 

 ground. More frequently the fallen fruits hang between the entangled stems, &c., 

 of other plants lying on the ground, as shown in fig. 147 4 . Here again the arm is 

 fixed, and the thicker, lower end is set in motion. The movement may be compared 

 to that of an augur, although in consequence of the swaying and alteration in 

 position of the beak, unavoidable in windy weather, see-sawing movements occur 

 in the boring part, and these are apparently advantageous. Like the fruits of the 

 Feather-grass those of the Stork's-bill are beset above the sharp point with erect, 

 stiff hairs. These hairs also play the same part as in the Feather-grass. Accord- 

 ing to the species various divergencies are found in the fruits of Feather-grasses 

 and Stork's-bills. The twisting of the lowest portion of the awn generally differs 

 from that of the knee-like bent part; the bristles on the glumes are sometimes 

 arranged in two longitudinal rows and sometimes they form a ring below and 

 are continued upwards as a one-sided longitudinal stripe, and so forth. Many 

 species of Stipa have no plume to the awn, and approximate closely in form 

 to the fruit of the Stork's-bill. The same remark applies to fruits of the genera 

 Aristida and Heteropogon, which are allied to Stipa. But in the main all these 

 developments agree with one another. The aim and object of the wonderful 

 mechanism just described is not so much the penetration of the pericarp or seed- 

 husk to a great depth into the soil, as the fixing of it firmly in the germinating bed. 

 It still remains to be pointed out that the cotyledons are only withdrawn with- 

 out delay from their investments when the latter are firmly cemented, anchored, or 

 fixed in some way to the substratum. When this is not the case, it often happens 

 that the fruit, or seed-coat, is raised up like a cap by the enlarging cotyledons. 

 The pull, otherwise exerted by the elongating hypocotyl, cannot under these 

 circumstances assist the cotyledons in their liberation. Often, indeed, the cotyle- 

 dons throw off the seed-coat unaided, but this is not always the case. In many 

 instances their apices remain squeezed up in the cavity of the husk, stunted and 

 yellow, and this reacts injuriously on the seedling, often causing weakness and 

 even death. It is therefore a mistake for gardeners to plant seeds in loose earth 

 where no good hold is afforded, since then, on germination, the seed-coats are raised 

 up by the only half -liberated cotyledons, whose apices are still imprisoned. 



