EXOGENOUS STRUCTURE. 115 



renchyma. But as soon as it begins to grow, while the cotyledons 

 only are developing (as in Fig. 106, 107), some of the cells begin to 

 lengthen into tubes, to be marked with transverse bars or spiral 

 lines, and thus give rise to ducts or vessels (5T-60); these are 

 grouped as they form into a small and definite number of bundles 

 or threads, say four equidistant ones in the first instance, as in the 

 Sugar Maple : other slender cells of smaller calibre, and destitute 

 of markings, soon appear surrounding the threads of vessels, and 

 forming the earliest woody tissue. As the rudiments of the next 

 internode and its leaves appear, two or four additional threads of 

 vascular tissue appear in the stem below, in the parenchyma be- 

 tween the earliest ones, and equally surrounded with forming 

 woody tissue. At an early stage, therefore, the developing stem 

 is seen to be traversed by several bundles of woody tissue with 

 some vessels imbedded ; and these, as they increase and enlarge, 

 run together so as to make up a woody sheath, or, as seen in the 

 cross-section, a ring, inclosing the central part of the parenchyma 

 within it, and itself inclosed by the external parenchyma. Thus 

 a circle or layer of wood is formed, which is in such a way im- 

 bedded in the original homogeneous cellular system as to divide it 

 into two parts ; namely, a central portion, which forms the pith, 

 and an exterior zone, which belongs to the bark. The whole is of 

 course invested by the skin or epidermis, which covers the entire 

 surface of the plant. The way in which the layer of wood thus 

 originates is somewhat rudely illustrated by the annexed diagrams 

 (Fig. 151 - 153). The several woody masses, especially in trees 

 and shrubs, are separated from each other by lines or bands of the 

 original cellular tissue which pass from the pith to the bark, and 

 which necessarily become narrower and more numerous as the 

 woody bundles or wedges increase in size and number. These 

 are the 



191. Medullary Rays, which form the radiating lines that the 

 cross-section of most exogenous wood so plainly exhibits, espe- 

 cially that of the Oak, Plane, &c. They are the remains of the 

 cellular system of that part of the stem, condensed by the pressure 

 of the woody wedges, or plates, and which serve to keep up the 

 communication between the pith and the bark. 



192. The First Year's Growth of an exogenous stem accordingly 

 consists of three principal parts ; namely, 1st, a central cellular 

 portion, or Pith ; 2d, a zone of Wood ; and 3d, an exterior cellular 



