LESSON 19.] 



THE EXOGENOUS STEM. 



53 



die down annually, consists of a central cellular pith, a circle of vas- 

 cular bundles in the form of wedge-shaped masses, an external bark, 

 with its integumentary covering, and rays connecting the pith to the 

 bark. 



312. In stems which are not annual, the growth of the second 

 year consists of a new formation of vascular bundles outside the pre- 

 viously formed layer, between it and the bark. 



313. Between the pith and bark also, there are annually formed 

 a layer of active, formative cells, called cambium cells, which are 

 concerned in the development of new woody fibres. In illustration a 

 transverse section of the maple 



(Fig. 88, A) is shown, at the com- 

 mencement of its second year's 

 growth ; a longitudinal section of 

 the same plant (B) is combined 

 with the former, and the letters of 

 reference apply to the like tissue, 

 in both sections. 



314. The layer of spiral vessels 

 which surround the pith, and con- 

 stitute what is called the medulla- 

 ry sheath, are seen at a ; the me- 

 dullary rays pass through this 

 layer, at different points, to the 

 bark ; at b are the porous or pitted 

 vessels, presenting (in A) large 



rounded openings ; c, c, fibres formed of fusiform tubes, the letter to 

 the right hand marking the fibres forming the wood of the stem, and 

 the letter to the left, those which form the cortical fibres of the inner 

 bark ; d, cambium cells, between the wood and bark ; e, e, cortical 

 cells, often of a greenish color, forming a cellular layer of bark ; /, 

 outer cellular layer of bark, composed of cubical colorless cells, often 

 of a corky nature, and hence called suberous ; this cortical layer is 

 covered by the general integument, or epidermis. 



315. Thus, at the commencement of the second year's growth, 

 there is a distinct formation of cambium cells, by the action of which 

 a new layer of wood and a new layer of fibrous bark is formed, and 

 these cambium cells, being in connection with the medullary rays, 

 keep up the connection between the medullary and cortical cells. 



316. It is exceedingly interesting and instructive to notice the 

 changes that take place in the permanent woody stem of the exoge- 



Exogenous wood. Maple. 



