ENDOGENS. 97 



circumference. When there Is no limited circumference assigned by nature 

 to an Endogcn, then the curved spots, Avhich are sections of the woodv arcs, 

 are much more equally arranged, and are less crowded at the circumference. 

 NeA'er is there any distinct column of pith, or medullary rays, or concentric 

 arrangement of the woody arcs ; nor does the cortical integument of the 

 surface of endogenous stems assume the character of bark, separatino- from 

 the wood beloAv it ; on the contrary, as the cortical integument consists very 

 much of the finely divided extremities of the woody arcs, they necessarily 

 hold it fast to the wood, of which they are themselves prolongations, and 

 the cortical integument can only be stripped off by tearing it away from the 

 whole surface of the wood, from which it does not separate without leaving 

 myi'iads of little broken threads behind. 



This is the apparent and general structure of the most perfect among 

 Endogens, It is of course modified exceedingly' according to the nature of 

 particidar individuals, and may even be reduced to nullity, as is the case in 

 Lemna, Tillandsia usneoides. Naiads, and similar plants. 



Schleiden, who treats this subject in a merely anatomical manner, thus 

 describes the peculiarities of Endogens or Monocotyledons, and the manner 

 in which they differ from Exogens or Dicotyledons. 



In all plants, he says, the woody bundles, whose development always 

 proceeds from the interior to the exterior, are either limited or unlimited 

 in their growth. Commonly every woody bundle consists of three different 

 physiological parts ; firstly, of a tissue of extreme delicacy, capable of rapid 

 development, in which new cells are continually generated and deposited in 

 various ways, in two different directions, viz. next the circumference, in the 

 shape of a peculiar kind of lengthened cellular tissue with very thick walls, 

 the liber ; and next the centre, in the form of annular, spiral, reticulate, 

 and porous vessels : secondly, of woody cells, which are either uniform in 

 appearance, or different, and fomi the wood, properly so called. Up to a 

 certain period the development of the vascular system in Monocotyledons 

 and Dicotyledons proceeds upon the same plan ; but in Monocotyledons (En- 

 dogens) the active, thin, solid, delicate cellular tissue, suddenly changes ; the 

 partitions of its cells become thicker ; their generating power ceases ; and 

 when all the smTounding cells are fully developed, they assume a peculiar 

 form, ceasing to convey gum, mucilage, and other kinds of thick formative sap. 

 From this cause all further development of vascular bundles is rendered 

 impossible, and therefore Schleiden calls the woody bundles of such plants 

 '' limited." In Dicotyledons (Exogens), on the contrary, this tissue retains, 

 during the whole lifetime of the plant, its vital power of formation ; conti- 

 nues to develop new cells ; and so increases the mass, ceaselessly augment- 

 ing both the exterior (liber), and the interior faces (wood), for which 

 reason Schleiden calls such woody bundles "unlimited." This, he con- 

 tinues, happens according to the climate and nature of the plant : either 

 pretty continuously, as in Cactaceae ; or by abrupt periodical advances and 

 cessations, as occurs in forest trees of Europe. In the latter, the stem forms 

 an uninterrupted tissue, from the pith to the bark, during every period of 

 life, and the bark is never organically separate from the stem ; what is con- 

 sidered their natural separation in the spring, is only a rent produced by 

 tearing the delicate tissue already spoken of, which is present, even during 

 winter, and constitutes the foundation of new annual zones, although com- 

 pressed, and filled with gum, starch, and other secretions. In the spring, 

 being expanded and swollen by the new current of sap, it is deprived of its 

 contents by their solution. 



