CHAPTER III. ORGANS OF PLANTS. 183 



the root, save that it commonly possesses stomata. In woody 

 stems it is usually soon ruptured and destroyed by the growth of 

 the sublying tissues of the cortex, but in herbaceous stems it 

 commonly persists during the life of the organ. 



The primary cortex usually consists of chlorophyll-bearing 

 and other parenchyma, but it may also contain collenchyma, 

 sclerotic, fibrous, laticiferous and other tissues, as well as secre- 

 tion reservoirs. Frequently the outer portion becomes developed 

 into a hypoderma, consisting of collenchyma, sclerotic or fibrous 

 tissues. 



The central cylinder soon develops in its interior one or more 

 fibro-vascular bundles. The character of the stem and its mode 

 of growth will depend very largely on the character and arrange- 

 ment of these primary bundles. 



In the lowest plants that show a clear differentiation between 

 stem and leaf, the structure of the stem is very simple. In the 

 stems of mosses, for example, no fibro vascular bundles, properly 

 so-called, are found, but in many instances the interior tissues 

 of the stem consist of considerably elongated cells forming a 

 mass of conducting tissue which may be regarded as anticipating 

 the fibro-vascular bundles of higher plants. 



Among vascular plants, three principal types of stem-struc- 

 ture may be distinguished : the Fern type, the Palm or Monoco- 

 tyledon type, and the Dicotyledon type. Within the limits of each 

 type considerable variations occur. There are also exceptional 

 forms of rare occurrence that it would be difficult to include 

 in any of these types. 



The Fern Type. This is the one prevalent in the Ferns and 

 their allies, in which the bundles are usually of the concentric 

 kind, consisting of xylem surrounded by phloem. Occasionally, 

 however, they are of the radial variety. In some cases there may 

 occur within the central cylinder but a single bundle, from which 

 branches are given off to the leaves ; in others, there may be 

 two or more bundles placed side by side ; and in still others, and 

 this is the commonest arrangement, they are disposed in a single 

 circle. But when distributed circularly, they are never radially 

 elongated, as are the bundles in the stems of Dicotyledons, and 

 if viewed in transverse section they appear either circular, or 

 more or less lengthened in a tansrential direction. Interior to 



