Page:Proceedings of the Royal Society of London Vol 1.djvu/75

Rh In all kinds of wood, he says, there are two sorts of grain,-the false or bastard, and the true or silver grain. The former consists of the concentric circles which mark the annual increase of the tree; and the latter is composed of thin laminæ, diverging in every direction from the medulla to the bark, with different degrees of adhesion to each other at different seasons, and lying between and pressing on the sap-vessels of the alburnum.

If these laminæ are expansible under various changes of tempera ture, or from any other cause arising from the powers of vegetable life, our author conceives that they are as well placed as is possible to propel the sap to the extremities of the branches. That they are affected by the changes of temperature in the air is proved by the effects of these changes on them even after the tree is dead, as in the instance of boards, which warp more or less, according to the direction of this grain: and other instances are given of the effects of solar heat on different parts of plants, which materially favour this assertion.

The general conclusions derived from these experiments are, That the tubes of the alburnum, acted upon by the agency of the silver grain, are in fact the channels which, extending from the extremities of the roots to the points of the annual shoots, convey the nutricious juices to the base of the buds, and in the soft and succulent part of the annual shoot, where the alburnum with the silver grain ceases to act, and where commences the action of the central vessels, with their appendages the spiral tubes, that having through these reached the end of the leaves, the sap undergoes a change, perhaps from the action of the atmosphere, and is then brought back again through the external vessels of the leaf-stalks to the bark, which conveys it to every part of the tree, and ultimately contributes to its growth. In speaking of the use of the medulla, the author assigns his reasons for considering it as a reservoir of moisture, which it occa- sionally imparts to the leaves and fruit through the central vessels, and which these organs must often stand in need of, as they cannot, like animals, resort to the brook or shade. The heart or coloured wood of the trees he considers as the bones in the animal ceconomy, being intended to support them against the effects of winds and other destructive agents; and, accordingly, it is not found in roots or tender shoots, but is only formed when the vegetable has acquired & bulk which renders such a structure necessary.

This may be considered as a supplement to Dr. Herschel's paper on the nature of the sun, lately read to thee Socicty, and consists