2G6 On the P/anthig of For est -Trees. 



are filled up with a vascular or pulpy green substance, similar to 

 that beneath the transparent membrane of the bark, and the whole 

 is covered by the same kind of skin, furnished with almost innu- 

 merable pores, chiefly on the under side, which serve the purpose 

 of transpiration, or of communication between the air and the 

 internal part of the leaf. I have observed that the coating of the 

 leaf is colourless and transparent, and therefore it is the hue of 

 the pulpy or vascular matter peculiar to each plant, which imparts 

 that agreeable variety of green so remarkable, in the vegetable 

 kingdom. The calyx and corolla of the blossom have a similar 

 construction to that of the leaf; but a particular description of its 

 several parts is not essential to the present object.* 



As the temperature of the spring increases, the spongioles of 

 the fibrous roots expand, and absorb the surrounding water, 

 which contains in solution carbonic acid gas and common air, 

 with minute portions of alkaline and earthy salts, such as the soil 

 affords. This incipient sap, passing through the larger branches 

 of the roots and the alburnum of the stem to the extremities of 

 the tree, carries with it the elaborated matter, or true sap, which 

 had been deposited in the cells or interstices of those parts in the 

 previous autumn, and which, now mixed with the new sap, serves 

 to develop the leaves and bloS^soms, the buds of which had been 

 formed in the previous summer. I must here observe, that the 

 sap-vessels communicate also sideways with each other, so that 

 when a branch is cut off, the sap which otherwise w^ould have gone 

 into it receives a new direction. Just bef^ore this development of 

 the leaves takes place, if an incision be made into the alburnum, 

 or sap-wood,, the sap exudes copiously ; but as soon as the leaves 

 are opened this bleeding ceases, in consequence of the sap being 

 determined towards them. In the leaves, when fully developed, 

 the sap which continues to ascend comes in contact with the ex- 

 ternal air by means of the pores already described, through which 

 it receives carbonic acid gas ; by the action of light upon the 

 leaves, the sap undergoes a curious elaboration, gives off its re- 

 dundant moisture and oxygen gas, and becomes what is called 

 cambium, or true sap, which descends through the vessels of the 

 bark to be deposited as a new layer of wood, and to form new bark 

 and new roots. In the autumn a change takes place in the des- 

 tination of the descending sap, and it then no longer forms wood, 

 but is deposited in the cells of the alburnum and of the roots, 

 which serve as reservoirs to perform its part in the ensuing spring 

 as before described. In the winter, therefore, the cells of the 

 alburnum and the roots are swollen with this elaborated sap, con- 



* For a more particular description of the several organs, I refer the 

 curious reader to the 'Elements of the Organs of Plants,' by Decandolle 

 and Sprengel. 



