BOTANY. 455 



line substances, and saccharine and colouring matter. It flows 

 through the vessels, and particularly those which surround the 

 pith, is elaborated in traversing the numerous canals, mixed 

 with the proper juices of the vegetable, and contributes to the 

 growth of its various parts. This sap is most plentiful in 

 spring, when it is drawn upwards for the production of the 

 leaves and fruit ; and in autumn, when the period of flowering 

 is past and the leaves begin to fall, in its descending progress. 

 The transpiration of vegetables through the medium of their 

 leaves is very great ; and in equal masses in the same time it 

 has been calculated that they transpire seventeen times more 

 than the human body. Besides water, vegetables exhale oxy- 

 gen by their green parts in the light, and carbonic acid gas in 

 the shade. The water condensed by cold may often be seen 

 in drops of dew at the extremities of the exhalant vessels. Dur- 

 ing day the leaves transpire most on account of the heat and the 

 evaporation of their fluids, and the sap is then attracted chiefly 

 by the roots. In the night the leaves absorb most, and the sap 

 descends towards the root. Absorption is performed by the un- 

 der surface of the leaves, transpiration by the upper surface. 



The sap elaborated in the organic tissues composes a vege- 

 table gelatinous exudation developed between the bark and al- 

 burnum. This substance, which has received the name of 

 cambium, is analogous to the exudation which issues from 

 the vessels of animal bodies to unite divided parts or join a 

 wounded surface. The principal destination of this cambium 

 is to form the inner bark or liber, as it was called by the an- 

 cients, from being used for the purpose of writing. Gradually 

 thickening between the bark and the wood into a membrane 

 or tissue, it becomes attached to the alburnum, or outer and 

 softer portion of the wood, and in trees increases by annual 

 layers. A new layer of cortical vessels is also every year ad- 

 ded to the bark. Thus, as a tree is so many years old, an 

 equal number of these annual rings occurs ; and the age of trees 

 may be reckoned from counting the number of the annual lay- 

 ers. The layers of wood next the central pith or medulla in Di- 

 cotyledonous plants are hardest from the compression of the an- 

 nual circles ; and the net-work of vessels of the inner layers be- 

 comes nearly obliterated, and unable to conduct the vital juices. 

 Thus aged trees begin to decay in the centre, while, as in many 

 very old oaks, the outer portion of the trunk is vigorous and 



