File:The causes and course of organic evolution; (1918) (14587326589).jpg

a, plumule uniformly exposed to light;

b, plumule of plant placed transversely in dark box with light admitted by slit s;

c, plumule of upright plant in box.

The shoots of b and c show resultant response, in varying degree, to apogeotropic, apohydrotropic and heliotropicstimuli.

Identifier: causescourseofor00macf (find matches)
Title: The causes and course of organic evolution;
Year: 1918 (1910s)
Authors: Macfarlane, John Muirhead, 1855-1943
Subjects: Evolution
Publisher: New York, The Macmillan Company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Text Appearing Before Image:
Here them in the above three cases the same type of radicle. when acted by the same stimuli, applied at different angles, starts an intrinsic molecular machinery in order to plan out a pathway that is the most satisfying for each to pursue, but that yet differs in all three, and that is the compounded resultant of all three environal forces, acting on or stimulating the young cells of the radicle. The three radicles proenviron (to coin a suitable verb) exact and yet different pathways, before even the slightest external indication is shown. As every plant and animal physiologist knows, the length of time needed for proenvironing determination has been termed the excitation or latent period. The future "fate for weal or woe" of the plant may largely depend on the proenvironed pathway projected by the protoplasmic or chromatin molecules that constitute the energids.

But let us pursue the investigation further. As the plumule or young stem disengages itself from the seed coat, in an open exposed situation, it at once plots out a proenvironal course
Text Appearing After Image:
Fig. 4.—Seedlings of pea. a, plumule uniformly exposed to light; b, plumule of plant placed transversely in dark box with light admitted by slit s; c,plumule of upright plant in box. The shoots of b and c show resultant response, in varying degree, to apogeotropic, apohydrotropic and heliotropicstimuli.

that ordinarily is apogeotropic, positively heliotropic, andapohydrotropic. So the actual pathway pursued is as seen in Fig. 4, a. But the second, that was placed under a belljar, pursues the course of Fig. 4, b, while the third develops a direction as in Fig. 4, c. Further, when the fully formed green leaves have arisen on each, it can readily be shown that these move their surfaces, or even in many flowering plants their entire leaves, through pathways that are compounded resultants due to optimum lumic stimulus (heliotropism), to gravic action (diageotropism), to supra maximum light action or intensity (paraheliotropism), even in some cases to to mechanic and chemical stimuli.

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