THE PITH OF ENDOGENOUS PLANTS. 89 . 



from either end of the arc about one foot and a-half. It is not presumed that all the 

 fibres enter the bark, but that some curl and attach themselves to it, whilst others pass 

 to the roots. Thus the perpendicular section of a Palm would exhibit a series of arcs, 

 intersecting each other, and originating in points gradually ascending as the tree grew 

 in height. These arcs would proceed from every point of the circumference of the 

 stem, and present their convexities towards the centre. , 



Thus far, the best observers are agreed ; but the precise point of origin is still a 

 matter in dispute. That they proceed from foliaceous organs, as in exogens, is certain ; 

 but whether from the fully-developed leaf, or from undeveloped interfoliar organs, is 

 undetermined. The former is the opinion of Mohl, and the latter of Schleiden ; whilst 

 Mirbel occupies a midway position, and asserts that they proceed from an independent 

 part of the growing point, or Phyttophore. The successively ascending points of origin 

 of the arcs are explained on either of these theories ; for in endogens the foliaceous 

 organs, whether developed or undeveloped, are placed only on the top or head of the 

 stem, and are yearly supplanted by others rising from above them as the stem 

 elongates. 



There is, however, a material difference of opinion as to the immediate direction 

 taken by the growing wood, according to the views above expressed. Thus, on Mohl's 

 theory, the woody bundles aro formed at the highest point viz. within the leaf itself, 

 and have but one direction, that from above downwards ; but on Mirbel' s theory, the 

 point of origin is below the leaf, and the bundles pass in two directions one upwards 

 into the leaf, and the other downwards into the stem. If we adopt the latter theory we 

 must admit that the oldest part of the wood is neither at the top nor at the bottom of 

 each bundle, but at an intermediate spot a point near to the upper extremity. 



It is agreed by all observers, that there is no evident dissimilarity between an 

 exogenous and endogenous stem up to the end of the first year of growth ; for in both 

 cases there is a central pith and an external layer of wood, which has been divided 

 into two portions, one of which has applied itself to the bark, and become the liber ; 

 whilst the other is the true wood, which surrounds the centre. The distinction begins 

 in the following year, when the divisions of the bundles of woody fibre into liber and 

 wood does not again take place in endogens ; and, as in those plants, the whole wood 

 passes down into the bark, and near to it, the lower part of the stem, as in palms, is 

 much more solid and resisting than the upper part. 



The uses of vascular tissue in endogens are the same as those of exogens, but the 

 proportion of saccharine juices which it contains is greater in the former than in the 

 latter. This is a beautiful arrangement for the convenience of those inhabiting hot 

 and often arid countries, where animal milk and water are but sparingly afforded. (See 

 page 24.) 



Pith. The pith in endogens may be said to occupy the whole of the stem, and to 

 form the bed into which the woody fibres pass. If we deny the existence of bark in 

 endogens, we must affirm that the pith also forms the cuticle, having first had its 

 cells thickened to render it more resisting. On the section of a stem it will therefore 

 be found to intervene between the bundle of vascular tissue, as exhibited in Fig. 146, 

 and to form the very boundary to the stem itself. In the endogenous plants of our 

 climate, as the grasses, and in many similar fast-growing specimens of hot countries, 

 as the bamboo, the central pith is ruptured, and ultimately absorbed ; so that there is a 

 central vacuity, except at the nodes, where a partition of pith still continues. 



The uses of pith are chiefly two : first, to supply a soft, elastic, and yielding struc- 



