108 



SCIENTIFIC AGRICULTURE. 



Fig. 17. 



product of the first year's growth; 

 one new layer is formed every suc- 

 ceeding year, so that the number 

 of rings or "grains" at the base of 

 the stem indicate correctly the age 

 of the tree. Each layer is composed 

 of woody fibres, vasiform tissue and 



ducts. 



Fig. 17. 



[Fig. 17. 1, represents an erogenous 

 stem of one year's growth; a, pith; b, 

 bark; c. medullary rays; d, woody bun- 

 dles of fibre; 2, laticiferous vessels of the 

 bark.] 



The outside, lighter colored lay- 

 ers constitute the allurmnn or "sap 

 wood:" the brownish layers inside are harder than the sap 

 wood, and are hence called the duramen. 



The bark forms the external covering or integuments of the 

 stem and root. The bark consists of three distinct layers : the 

 outside covering is called the epidermis, this layer is some- 

 times covered with a coating of gummy, oily or resinous matter. 

 The middle layer is the cellular integument; and the inner coat 

 the liber. The two outer layers are of cellular structure, "while 

 the inner one is both cellular and woody. 



The sap is carried by the vessels through the alburnum to 

 the leaves, with the vessels of which they communicate: while 

 in the leaves, the sap undergoes some changes, (not well 

 understood,) by moans of the air and light, by which it is 

 converted into a fluid called latex. From the vessels of the 

 under side of the leaf, it descends by the vessels of the inner 

 bark ; part of it is carried inwards by the pores of the medul- 

 lary rays, and diffused through the stem, while the remainder 

 descends to the roots, and is distributed through them. Sap 

 is milky, gummy, saccharine bitter, &c., in various plants. 



At the end of spring a portion of the descending sap, which 



