. The causes and course of organic evolution; . li, applied at different angles,starts an intrinsic molecular machinery in order to plan outa pathway that is the most satisfying for eacli to pursue, butthat yet differs in all three, and that is the compounded resultant Active Causes of Organic Evolution 191 of all three enmronal forces, acting on or stimulating the youngcells of the radicle. The three radicles proenviron (to coina suitable verb) exact and yet different pathways, before eventhe slightest external indication is shown. As every plantand animal physiologist knows, the length of ti
. The causes and course of organic evolution; . li, applied at different angles,starts an intrinsic molecular machinery in order to plan outa pathway that is the most satisfying for eacli to pursue, butthat yet differs in all three, and that is the compounded resultant Active Causes of Organic Evolution 191 of all three enmronal forces, acting on or stimulating the youngcells of the radicle. The three radicles proenviron (to coina suitable verb) exact and yet different pathways, before eventhe slightest external indication is shown. As every plantand animal physiologist knows, the length of time neededfor proenvironing determination has been termed the excitationor latent period. The future fate for weal or woe of theplant may largely depend on the proenvironed pathway pro-jected by the protoplasmic or chromatin molecules that con-stitute the energids. But let us pursue the investigation further. As the plumuleor young stem disengages itself from the seed coat, in an openexposed situation, it at once plots out a proenvironal course. Fig. 4.—Seedlings of pea. a, plumule uniformly exposed to light; h, plu-mule of plant placed transversely in dark box Avith light admitted by slit s; c,plumule of upright plant in box. The shoots of b and c show resultant re-sponse, in varying degree, to apogeotropic, apohydrotropic and heliotropicstimuli. that ordinarily is apogeotropic, positively heliotropic, andapohydrotropic. So the actual pathway pursued is as seenin Fig. 4, a. But the second, that was placed under a bell-jar, pursues the course of Fig. 4, b, while the third devel()])sa direction as in Fig. 4, c. Further, when the fully formed 192 Causes and Course of Organic Evolution green leaves have arisen on each, it can readily be shoTMi thatthese move their surfaces, or even in many flowering plantstheir entire leaves, through pathways that are compoundedresultants due to optimum lumic stimulus (heliotropism), togravic action (diageotropism), to supra maximum light actiono
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