55 



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 laminae, diverging in every direc- 

 tion 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 laminae 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 A r essels, 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 

 a bulk which renders such a structure necessary. 



Additional Observations tending to investigate the Symptoms of the 

 variable Emission of the Light and Heat of the Sun ; with Trials to 

 set aside darkening Glasses, by transmitting the Solar Rays through 

 Liquids ; and a few Remarks to remove Objections that might be 

 made against some of the Arguments contained in the former Paper. 

 By William Herschel, LL.D. F.R.S. Read May 14, 1801. [Phil. 

 Trans. 1801, p. 354.] 



This may be considered as a supplement to Dr. Herschel's paper 

 on the nature of the sun, lately read to the Society, and consists 



