ITS INTERNAL STRUCTURE. 113 



its diameter. Into this the elongated cells that form the woody 

 tissue and ducts are introduced vertically ; they run lengthwise 

 through the stem and branches. Hence, the latter has been called 

 the longitudinal, vertical, or perpendicular system (56, 64) ; and 

 the cellular part, the horizontal system of the stem. Or the stem 

 may be compared to a web of cloth ; the cellular system forming 

 the woof, and the woody, the warp. It will be seen hereafter, that 

 this illustration not inaptly represents the real structure of the stem. 



184. The diversities in the internal structure of the stem are 

 principally owing to the different modes in which the woody or 

 vertical system is imbedded in the cellular. These diversities are 

 reducible to two general plans ; upon one or the other of which 

 the stems of all Flowering Plants are constructed. Not only is the 

 difference in structure quite striking, especially in all stems more 

 than a year old, but it is manifested in the whole vegetation of the 

 two kinds of plants, and indicates the division of Phsenogamous 

 plants into two great classes, recognizable by every eye ; which, in 

 their fully developed forms, may be represented, one by the Oak 

 and the other trees of our climate, the other by the Palm (Fig. 166). 



185. The difference between the two, as to the structure of their 

 stems, is briefly and simply this. In the first, the woody system is 

 deposited in annual concentric layers between a central pith and 

 an exterior hark ; so that a cross-section presents a series of rings 

 or circles of wood, surrounding each other 



and a distinct pith, and all surrounded by 

 a separable bark. This is the plan not 

 only of the Oak, but of all the trees and 

 shrubs of the colder climates. In the 

 second, the woody system is not disposed 

 in layers, but consists of separate bundles 

 or threads of woody fibre, &c., running through the cellular 

 system without apparent order ; and presenting on the cross-sec- 

 tion a view of the divided ends of these threads in the form of dots, 

 diffused through the whole ; but with no distinct pith, and no bark 

 which is at any time readily separable from the wood. The ap- 

 pearance of such a stem, both on the longitudinal and the cross- 

 section, is shown in Fig. 150 ; it may also be examined in the Cane 

 or Rattan, the Bamboo, and in the annual stalk of Indian Corn or 



FIG. 150. Section of a Palm-stem. 



10* 



