THE STEM. 39 



which consists of tubes and sometimes of cclluhir tissue. If 

 we commence our ohsorvation from tlie pith, we sluill ol)serve 

 immediatelv surroundin«r it a very thin greenish hiycr, called 

 the mciiulliirv sheatli, represented in F'i^. 15, /'. Immechately 

 1^5 ^^_^^ in contact with tliis is a zone of cel- 



^^^^^^ ^**^>^ hilar substance and tubes, as seen 



^^^^iT" / / >^^^'" ^'''?^* ^^' ^' ^^^^ ^^ ^'''^ ^ dense 



^V-^\V^^ ?Llf/// woody zone as seen Fig. 15, h. The 



N^V^I jT/yf ^^^^ t^^o layers are deposited annu- 



/i--^<\yy[//^^^ ^"y* ^"^ always in the order laid 



9"^^SU^ down ; so that if we begin to reckon 



/'' \/ tVom the medullary sheath, we lind 



first the cellular substance and then the woody layer, and so 



on to the bark ; the cellular always commencing and the 



woodv layer always ending the scries. 



45. The jneduUiiri/ sheath is composed of spiral vessels and 

 woodv tibre connected by cellular tissue. It precedes every 

 other tbrmation except the cellular, in the elongation of 

 branches. The leaves derive their origin from the medullary 

 sheath. The true spiral vessels are found in the stem of ex- 

 ogenous plants only in the medullary sheath. In paragraph 

 25 we gave our opinion as to the reason tor spiral vessels 

 being tound only in this organ. They arc certainly re- 

 quired in the earliest development 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 i)hysical properties enable it to perform, is 

 performed by otiier varieties of tissue ; and to our mind there 

 is not a more beautiful instance of adaptation and design, or a 

 clearer illustration of Infinite wisdom in the constitution of 

 the vefietable kingdom, than is exhibited in the structure of 

 the organ under consideration. 



4G. If we examine our transverse section again, we shall 

 perceive that the woody part is separated apparently into nu- 

 merous wedge shaped portions, their bases terminating in the 

 bark, and their apices in the pith, as seen in Fig. 15. By a lon- 

 gitudinal section made in the direction of these lines, we shall 

 find that they are plates of substance proceeding from the 

 pith and terminating in the bark. They are called medulla- 

 ry processes^ or meduUanj rays, and arc composed of cells in 

 the form of thin parallelopipeds. In the embryo, and in the 

 earliest development of the stem, the cellular substance of the 

 bark and pith, are in contact, but immediatelv vascular and 

 woody fibres are sent down, which pierce the cellular sub. 

 stance, dividing the mass of the pith from the parenchyma 



