170 ELEMENTS OF STRUCTOEAL BOTANY. 



the epidermal and fibro-vasoular systems. In the 

 3xogens it embraces the pith, the medullary rays, and 

 parenchyma generally. The collenchyma found just 

 beneath the epidermis, sclerenohyma occurring in different 

 parts, and laticiferous tissue are also constituents of the 

 fundamental system, as well as the cork cells already 

 referred to. In the monocotyledons ground-tissue in the 

 form of parenchyma fills the space between the closed 

 bundles of the stem ; while in many plants in which 

 abro-vascular bundles are not produced, the ground- 

 tissue constitutes the whole of the interior. 



304. In exogenous stems the wood developed from 

 the cambium is often different from that of the pritoary 

 bundle as developed from the procambium. Pines, for 

 example, have vessels in the primary xylem, but none in 

 the secondary, the latter being almost entirely made up 

 of the cells with bordered pits, already described. 



305. The bundles of the leaves are continuous with 

 bundles in the stem. Leaves appear at first as protuber- ' 

 ances on the side of the stem close to the growing point, 

 and the upper ends of the primary bundles almost at 

 the very beginning bend outwards towards the new 

 leaves, iSie lower part being continued down the stem. 

 In the monocotyledons these bundles first arch inwards 

 towards the centre of the stem, and then outwards and 

 downwards, thinning out as they descend. Hence, in a 

 cross-section (Fig. 229) the bundles appear more crowded 

 towards the circumference, and also smaller. Such a 

 stem is, therefore, found to be harder at the outside than 

 at the centre. 



