THE STEM. 29 



53 1 If we take the stem of the Oak of one year's growth, 

 and examine a transverse section of it with a microscope, we 

 shall observe the following appearances, as exhibited in Fig. 22, 

 which represents a wedge extending from the outside to the 

 center of the pith : p indicates the cells of the pith, being loose 

 and large at the center, more compact toward the medullary 

 sheath ; m points out the medullary sheath, composed mostly 

 of spiral vessels ; w id «?, the woody tissue formed the first year ; 

 ddd, dotted ducts, the largest vessels of the stem, the open 

 mouths of which are seen on that part of the figure represent- 

 ing the transverse section ; c points out the thin layer of cam- 

 bium, which is the generating layer ; b, the liber of the bark ; 

 t, the lactiferous vessels ; c e, the cellular envelope or meso- 

 phlceum ; e p, the corky or suberose layer ; e, the epidermis. 

 These are the products of one year. 



54 • The medullary sheath is composed of spiral vessels and 

 woody fiber connected by cellular tissue. It precedes every 

 other formation except the cellular, in the elongation of branches. 

 The leaves derive their origin from the medullary sheath. The 

 true spiral vessels in the stem of exogenous plants are found 

 only in the medullary sheath. In paragraph 29 we gave our 

 opinion as to the reason for spiral vessels being found only in 

 this organ. They are certainly required in the earliest develop- 

 ment of the vegetable, and no increase in length of vascular 

 vegetables ever takes place in which this form of the elementary 

 organs does not enter into its formation. And we very well 

 know that every function, except what its physical properties 

 enable it to perform, is performed by other varieties of tissue ; 

 and to our mind there is not a more beautiful instance of adap- 

 tation and design, or a clearer 

 illustration of Infinite wisdom Fig - 24 - 



in the constitution of the vege- 

 table kingdom, than is exhib- 

 ited in the structure of the 

 organ under consideration. 



55* If we examine our trans- 

 verse section again, we shall 



perceive that the WOOdy part Exhibiting the medullary rays. 



is separated apparently into 



numerous wedge-shaped portions, their bases terminating in the 



bark, and their apexes in the pith, as seen in Fig. 24 ; and a 



53. What shall we find in the examination of a stem of oak of one year's 

 growth ?— 54. Describe the medullary sheath ? What derive their origin 

 from it i — 55. Describe the medullary rays. 



