EFFECTS OF GROWTH AS SEEN IN MOVEMENTS OF PLANT-ORGANS. 15 



was 52°. Similiar differences, only varying slightly, occurred on a mown 

 lawn. 



These observations appear to suggest the cause of the long creeping 

 stems in sand by the sea as of Carex arenaria, Agropyrum (Triticum) 

 junceum, &c, in that the hot, upper layer of the sand may be the stimulus 

 to develop shoots which then grow horizontally below the surface ; this 

 suggestion is supported by the fact (recorded by Babington) that Hordeum 

 murinum var. arenaria, Bab., has " the lower part of the stem buried, 

 lengthened and rooting, thus appearing to creep in the sand of the sea- 

 shore." 



Experiments have shown that the shoots of cress seedlings curved 

 away from the source of heat whereas maize seedlings curved towards 

 it. The roots of hyacinths are thermotropic, but the leaves apothermo- 

 tropic (Vines). I found seedlings of wild cabbage were indifferent 

 to heat, when the pot containing them was in total darkness, the 

 temperature by the side next a fire being 100° F., and the opposite 

 side, 80°. The pot was 3 inches in diameter. 



Haptotropism and Aphaptotropism. 



So many cases of organs bending after being touched are described by 

 Darwin in his works on The Movements of Plants and Insectivorous Plants, 

 &c, that the reader is referred to them. With regard to root- tips, " Sachs 

 discovered that the radicle a little above the apex is sensitive and bends 

 like a tendril towards the touching object. But, when one side of the 

 apex is pressed by any object, the growing part bends away from the 

 object. . . . We are therefore led to suspect that the apex was sensitive 

 to contact . . . Some few stems bend towards a touching object, but no 

 case, we believe, is known of an organ bending away from one." * The 

 advantage of this power to a root penetrating the soil is obvious. 



Oethotropism. 



Besides phototropism, another kind of orthotropic motion is produced 

 by growth in the development of leaf-buds. If the opening bud of lime 

 or beech be observed in spring, the young leaves, as they appear, curve 

 downwards, and as they gradually enlarge and become full-grown, rise up 

 again and assume the horizontal position. In the walnut the petiole of 

 the pinnate leaf curves strongly downwards, and only rises when the 

 leaflets are fully developed. In the ash the petiole curls upwards. In 

 every case they are in a vertical line, hence I call it orthotropism.f 



The "object " of these movements is to place the delicate immature 

 blades in an erect or vertical position so as to avoid the chill produced by 

 radiation, which is always greater from a horizontal surface. Leaves 

 perform very similar movements in the tropics, but in this case it is to 

 protect them from desiccation under the powerful heat of the sun. The 

 movement, therefore, is apheliotropism. 



* Movements of Plants, pp. 131, 132. 



f " Vernation and the Methods of Development of Foliage, as Protective against 

 Radiation." — Jour. Lin. Soc., vol. xxi. p. 624. 



