PHENOMENA CONNECTED WITH GROWTH. 19 



A portion is returned to the atmosphere, the rest 

 remains to feed the plant. 



The carbonic acid of the air is also absorbed by 

 the leaves, unites with the matter drawn by the 

 spongioles from the soil, and is decomposed in the 

 tissue of the leaves, in order to become nutritious 

 matter. 



The sap, elaborated in this manner, takes the 

 name of cambium, and, changed in the nature 

 which it had during its ascent, it becomes proper 

 and suitable for the increase of the plant. The 

 sap ascends to the leaves through the outer layers 

 of the alburnum, or young wood, during the day. 

 It descends during the night to the spongioles by 

 the innermost layers (that is, by the newest formed) 

 of the liber, composing thus a new layer of lig- 

 neous fibre, and a new layer of liber. This is the 

 manner of growth in trees. There is, however, 

 this difference, that while the ascent of the sap 

 increases the length, the descent of the matured 

 sap, or cambium, increases the breadth of plants. 



In spring, the sap-vessels are already gorged 

 before the leaves show. Thence arises a pressure 

 on the axils of the buds, which precipitates the 

 young shoot into extension. The cambium de- 

 scending, on the other hand, solidifies the young- 

 shoot by means of the new layers which it inter- 

 poses. The cambium gives breadth as well as 

 b 2 



