242 MORPHOLOGY. 



myself endeavoured to throw additional light upon the origin of the 

 cork-layer.* The first formation of the periderma is still, however, very 

 obscure. 



The pith consists essentially, and very generally, of parenchyma, 

 without any manifestation of special layers in it. When old it becomes 

 either very thick-walled and porous, or it is destroyed, and then leaves 

 large air-cavities, as, for instance, in many Grasses, Umbelliferce, &c. 

 There frequently remain alternate, isolated, firmer layers of pith, and 

 thus form an air-cavity divided off into chambers placed horizontally one 

 upon the other, as, for instance, in Juglans regia. Interspersed in the 

 pith we find spiral cells, thick-walled porous cells, cells with peculiar 

 juices, milk-vessels, air-passages, and even, in the case of Rhizophora 

 Mangle, peculiarly-branched liber-cells, t In many of the woody Ro- 

 sacecB there are peculiar vertical and horizontal rows of very thick 

 porous cells, &c., in the pith. Innumerable plants yet remain to be in- 

 vestigated. Many more isolated phenomena might be noticed, but nothing 

 can be made of them at present. { Vascular bundles alone, with one 

 exception, are found in every axis, and, consequently, these are almost 

 the only parts of which the distribution and nature are susceptible of 

 general treatment. In the first place, we must mention one common dif- 

 ference affecting the nature of the vascular bundles, and their relative mass 

 compared with the cellular tissue of the axis. The vascular bundles either 

 preponderate in mass, and their elementary parts are generally very 

 much thickened, and the axis, having thus greater firmness, is termed a 

 woody stem or stalk ; or the vascular bundles are present only in a pro- 

 portionately small mass, separated from each other by larger quantities of 

 cellular tissue, and their component elementary parts, in a great degree, 

 or principally, have thinner walls, and then the whole mass of the axis is 

 softer, the thinner lax or still flexible, and the stem or stalk is termed 

 succulent : the last-mentioned is also, with superfluous diffuseness, called 

 herbaceous. In the succulent axes the course of the vascular bundles is 

 generally much more simple and regular, as may be seen in the different 

 internodes of the one and the same axis (which may be woody below and 

 succulent above). The distinction between a woody and a succulent axis 

 agrees still less with that of stem and stalk. We often find in the same 

 class some species having succulent, and others woody stalks ; and not un- 

 frequently a whole family have exclusively succulent stems. 



The arrangement and course of the vascular bundles are, however, the 

 most important points to be considered. In the main paragraphs I have 

 given the general features, but I will here enter somewhat more specially 

 into the question, as, at the same time, I separate the individual groups. 

 I add the following particulars as the general result of my own researches 

 on this subject, without, however, maintaining that they are to be received 

 as the ultimate expressions of a natural law : 



1. The origin of any vascular bundle, as well as the development of 

 one already extant, presupposes, without exception, the presence of a 

 cambium-layer, as every new formation of structure, in and upon the plant, 

 presupposes the existence of a process of cell-formation. By cambium, 

 cambium-layers, formative layers, we understand nothing more than 

 a cellular tissue, which has not yet ceased to develope new cells, in contra- 



* On the Cacti, loc. cit. 



f Wiegmann's Archiv. Jahrg. v. (1 839), part i. p. 232. 



j As, for instance, the remarkable gelatinous (?) masses, beset with crystals, lying in 

 special cells, found in the epidermis of Justicia, and in the bark and pith in Eranthe- 



