Page:The Effect of External Influences upon Development.djvu/61

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NOTE III (p. 13).

That the principle of intra-selection is at work in plants just as in animals is seen from other than merely general considerations. Vöchting has recently described some experiments with plants which were kept in a feeble light. These at first showed marked effects therefrom, producing, for instance, flowers that were much below the usual size; they afterwards, however, 'adapted' themselves to the abnormal illumination, and yielded flowers of the normal size. This adaptation is probably to be referred to intra-selection. (Compare Vöchting, 'Ueber den Einfluss des Lichtes auf die Gestaltung und Anlage der Blüthen,' Berlin, 1893, p. 11.)

Moreover, all the various adaptations of the parts of plants to the special influences that affect them, such as gravity, light, moisture, and chemical stimuli, must likewise be referred to the process of intra-selection. The delicate sensitiveness of tissue and protoplasm is hereditary, but it is through the struggle of parts that special adaptation to the actual conditions is secured. To take one example, already quoted above: the form of a tree is adjusted to the gap between other trees—vigorous growth takes place on the side next the light, while the shoots are retarded on the shaded side.

NOTE IV (p. 14).

Wilhelm Roux, Der Kampf der Theile im Organismus. Leipzig, 1881.

In this important treatise the principal subject discussed in addition to that of the spongy substance of bones is the form and branching of the blood-vessels in vertebrates. It is shown that these are constructed on the most suitable mechanical principles. For example, the lumen of a blood-vessel, where it leaves a larger trunk, is not of the usual cylindrical form, but is conical, and thus has the form which a free jet from a round lateral opening in the trunk would