NUTRITION. 43 



it is the same with the succeeding seasons, till 

 the period when the outer layer having acquired 

 by age the hardness of perfect wood, and no 

 longer admitting of further distension, is in- 

 capable of any increase of diameter. 



33. A Leaf has two distinct parts the Pe- 

 tiole, or stalk, and the Lamina, called also the 

 blade or limb ; the former consists of fibres pro- 

 ceeding from the stem, and enclosed in a cellular 

 integument ; the latter is formed by the ramifi- 

 cations of the fibres of the petiole, and the expan- 

 sion of its cellular tissue. In exogenous plants 

 the veins branch in various directions, so as to 

 form a kind of network ; in the endogenes they 

 run parallel to each other, and are simply con- 

 nected by transverse veins. When the petiole 

 becomes lengthened so as to curl up, it is called 

 a tendril, and many curious forms, such as that 

 of the Pitcher Plant, are but expansions of this 

 portion of the leaf. The limb of a leaf presents 

 three distinct parts; the superior and inferior 

 surfaces, and the mesophyllum, or substance con- 

 tained between the nervures. Both the surfaces 

 are ordinarily furnished with stomata, the under 

 side much more abundantly than the upper ; but 

 in leaves which rest by their under surface on 

 the water, this relation is reversed, their upper 



