692 



LECTURE XXXIX. 



over, the entrance of roots into the earth, and the driving in of the root-apex, 

 in which process the particles of soil must be pushed asunder, show that the 

 movement can by no means be compared to that of a flowing viscous mass ; the 

 root-apex, on the contrary, penetrates the earth much as does the point of a nail 

 hammered into a board, or as a worm does. In thus entering a heavy or co- 

 herent substance a point of application must of course exist : this is afforded in 

 Fig. 390 by the fixing of the seed with the needle. If such a point of application 

 does not exist the root-apex cannot penetrate into the mercury, but executes sinuous 

 movements on its surface. This is why it is so important that germinating seeds 

 should not simply lie on the surface of the soil, but should be covered with 

 a layer of earth sufficiently thick to enable them to offer sufficient resistance so as 

 to maintain their equilibrium whilst the root is penetrating into the deeper layers. 

 Germinating seeds which are not covered, or are too slightly covered, turn their root- 

 apices downwards, it is true, but they cannot penetrate into the earth, because the 

 small seed does not offer sufficient resistance. It is to be remarked, however, that 



Fig. 390.— a seedling of l'icüi Falm, the radicle and plumii 

 was then laid horizontally 1 



; of which had grov 

 I the surface of the 



: position : the seedlii 



seedlings possess manifold adaptations in order that even when they are not covered, 

 or insufficiently so, they may cling fast to the surface of the soil, so that the root- 

 apex obtains a point of application on entry ; this is often accomplished by means 

 of numerous root-hairs being developed very early and fixing themselves into the 

 soil. 



With respect to the phenomena of Heliotropism, I may treat of them much more 

 shortly than with respect to those of GeotrOpism, because, as we shall see immedi- 

 ately, they agree completely in all essential points : the effects of heliotropic 

 stimulation are exactly like those of geotropic, only the stimulus is a different one, 

 namely, Light. However, it is first necessary to describe generally the phenomena to 

 be here spoken of. In the case of plants growing in the open or in a garden 

 there is usually not much to be seen of heliotropic effects, especially if the 

 plants are nearly equally illuminated on all sides, as happens when the light is 

 reflected in all directions from clouds, &c. The direct light from the sun of course 

 produces unilateral illumination, but the source of light revolves, so to speak, around 

 the plant, and thus a similar effect is produced to that which I have already described 

 in connection with the apparatus on page 561. Nevertheless, in very sensitive plants, 

 e.g. young Flax-stems and flower-shoots of the Sunflower (the latter being named 



