134 SEAT OF VEGETABLE LIFE. 



The growth of young shoots and their components 

 has been already described ; here we are about to 

 attend to the manner in which this takes place. On 

 the approach of the vernal warmth, the first sign of 

 vegetation is observable in the swelling buds. At 

 this time the sap which, for the most part, remained 

 in a motionless and inspissated state during winter, 

 becomes fluid and readily flows from wounds made 

 through the bark. The narrow space between the 

 alburnum and liber contains a copious flow; the latter 

 appearing completely separated from the former. 

 Soon afterwards, as the season advances, shoots are 

 prolonged, and the foliage expanded ; the sap flows 

 less freely from wounds ; a new body becomes visible 

 in the space before mentioned, and begins to assume 

 a lymph-like state denominated cambium. In this 

 substance there is yet no appearance of organisation. 

 The organs are then so diminutive and colourless, 

 that neither their form, relative position, nor attach- 

 ments can be detected, even with the assistance of the 

 microscope. Mean time the bark is obviously giving 

 way to the expansive force of the swelling cambium, 

 and towards autumn, when the growth becomes lan- 

 guid, the cambium has acquired considerable consis- 

 tence ; the organisation is now visible, and very soon 

 assumes all the appearance of perfect alburnum. At 

 this time too, we find, that a new liber is separated 

 from the new body of alburnum, and remains closely 

 pressed against the liber of the former year. 



Now, whence have these new layers of wood and 

 inner bark derived existence ? They are not a dilata- 



