704 



LECTURE XL. 



of the cotyledon, the filamentous first seed-leaf. If the seed lies on or. in the 

 soil, so that the tip of the root is directed upwards by this elongation, as in the 

 figure [B), then it is not the root but the cotyledon which curves geotropically 

 downwards, so that the root-apex is compelled to penetrate into the soil (C and D). 

 Those as yet unacquainted with such matters may be not a little surprised 

 to see in the germination of the Water-nut {Trapa nutans), the tip of the root [w 

 in Fig. 396) of the seedling direct itself vertically upwards instead of penetrating 

 into the soil, but the anomaly is only apparent, for the part h does not belong to the 



primary root at all, but forms the so-called hypo- . 



cotyledonary segment of the stem, the rudiment 



of the root at w being completely aborted, and 



not having grown at all it is thus unable to 



accomplish any geotropic curvature downwards. . :; 



Moreover, there are other seedlings which 

 exhibit a similar phenomenon, when taken from 

 the soil and laid on a damp plate for instance. 

 If a portion of the plumule is already developed, 

 but the cotyledons still remain fixed in the seed, 

 and if the latter is heavy enough, the seed re- 

 mains quiescent in consequence of its great 

 weight, whereas, in consequence of the geotropic 

 up-curving of the seedling-stem, the root-apex 

 is carried up because it is lighter; in this case 

 of course if the primary root goes on growing 

 its end may curve downwards. Geotropism, 

 exactly like heliotropism, brings about only the 

 curvature of sensitive organs, as already de- 

 scribed; which end of the organ is to be 

 directed upwards in the process depends entirely 

 and simply upon the position of the fixed point. ': 

 These examples will sufiice to show that in 

 anisotropy we are concerned with one of the 

 most general properties of vegetable organiza- 

 tion; indeed it is quite impossible for us to 

 have any idea of how plants would look, or 

 could exist, if their various organs were not 

 anisotropous, and since their anisotropy is nothing other than the expression of 

 their different irritability to the influence of gravitation and light, with which, are 

 associated, in some cases, a sensitiveness for unequal distribution of moisture in 

 the environment, and probably some other less well-known influences, it is at once 

 obvious that it is the different irritability of the organs from which the external 

 configuration of plants in general arises — a statement from which I started in the 

 lectures on Organography. Even the most important differences in the biology 

 of plants are the expression of the different distribution of anisotropy, of which 

 innumerable examples may be quoted. To mention one only, the stem of Palms 

 generally grows vertically upwards, but there are also Palms {JSabat), the primaryi 



FIG. 396. — Germinating Water-nut (7Va/a nataiis). 

 /"coats of the fruit ; w aborted root ; h hypocotyl ; 

 c petiole of tlie large cotyledon which is buried in the 

 seed ; ef the small second cotyledon. Between the 

 two cotyledons is the bud of the primary shoot. 



