98 ENDOGENS. 



This difference between limited and unlimited woody bundles affords, in 

 Schleiden's opinion, the only universal distinction between Endogens and 

 Exogens. In the annual Exogens the unlimited woody bundle, checked in 

 its further development by the death of the plant, has, it is true, in so far 

 some similarity to the limited one of Endogens ; yet, sufficient research 

 shows the difference distinctly, for the formative layer in the former con- 

 stantly retains to the last moment its generating power. (See Annals of 

 Natural History, iv. 236.) 



The distinction between Endogens and Exogens, whether it be as we have 

 first described it, or such as Schleiden states., is so obvious and universally 

 recognised, that one would have thought them beyond the reach of contro- 

 versy. Nevertheless, M. de Mirbel has very recently (Comptes Bendus, Oct. 

 1844, p. 699) asserted, that, according to his theoretical views of their struc- 

 ture, a great number of Monocotyledons are Exogens, more especially Dra- 

 caena, Phoenix, Chamserops, and Bromelia. Meneghini, moreover, long since 

 pointed out the fact that Yucca gloriosa arranges its woody bundles in con- 

 centrical circles, (Ricerche sulla Struttura del Caule nelle Piante Monocoti- 

 ledoni, Padova 1836) and the same tendency is discoverable in some other 

 Endogens allied to Yucca. But the mere gathering together the woody 

 bundles into imperfect rings, does not in any degree invalidate the distinc- 

 tion between Endogens and Exogens, because their whole manner of growth 

 is different. The librovascular tissue which forms the wood of Yucca glo- 

 riosa itself, is in fact present in the form of arcs, just as much as in a 

 Palm-tree. 



In many of the larger kinds of Endogens the stem increases principally 

 by the development of a single terminal bud, a circumstance unknown in 

 Exogens, properly so called. In many however, as all grasses, the ordinary 

 growth takes place by the full development of axillary buds in abundance. 



In general there is so great a uniformity in the structure of an endoge- 

 nous stem, that the common cane or asparagus illustrates its peculiarities 

 sufficiently. There are, however, anomalous states that require explanation. 



Grasses are endogens with hollow stems strengthened by transverse plates 

 at the nodes. This is seen in the bamboo, whose joints are used as cases 

 to hold rolls, or in any of our indigenous species. In this case the devia- 

 tion from habitual structure is owing to the circumference growing faster 

 than the centre, the consequence of which is the tearing the latter into a 

 fistular passage, except at the nodes, where the arcs of ligneous tissue, con- 

 nected with the leaves, cross over from one side of the stem to the other, 

 and by their entanglement and extensibility form a solid and impenetrable 

 diaphragm. That this is so is proved by the fact, that the stems of all 

 grasses are solid, or nearly so, as long as they grow slowly ; and that it is 

 when the rapidity of their development is much accelerated that they assume 

 their habitual fistular character. In the sugar-cane grass the hollowness 

 of the stem is indeed unknown. Independently of that circumstance, their 

 organisation is sufficiently normal. 



Xanthorhaaa hastilis has been shown by De Candolle to have an anoma- 

 lous aspect. When cut through transversely, the section exhibits an 

 appearance of medullary rays proceeding with "considerable regularity from 

 near the centre to the very circumference. (Organographie Vegetale, t. vii.) 

 But such horizontal rays are not constructed of muriform cellular tissue like 

 real medullary processes, but are composed of ligneous cords lying across 

 the other woody tissue ; they are in fact the upper ends of the woody arcs 

 pulled from a vertical into a horizontal direction by the growth of the stem 



