Page:The American Cyclopædia (1879) Volume XIII.djvu/596

580 PLANT woody stems present at the end of the season ; in one oak this layer becomes unusually de- veloped, and is separated as cork, for which no substitute has as yet been found. Outside of the whole is the epidermis, a layer of thick- walled flattened cells. In a stem of this kind, when growth commences the second spring, the bark is readily separable from the wood, and between the two will be found a muci- laginous substance called the cambium; this consists of extremely delicate forming cells, as it is here that the growth of the stem takes place ; the inner portion of the cambium be- longs to the wood and the outer to the bark, and in this a layer of new wood forms outside of that of the previous year, while a layer of liber or inner bark is added inside of that of the year before, the medullary rays also elon- gating so that the wood of the second year is also traversed by these. The increase in the woody system of the stems of dicotyledons taking place always by additions on the out- side of the old wood, such stems are exoge- nous, or outside-growing, and the term exo- gen is used as synonymous with dicotyledon, even when the plant is only an annual. In ex- ogenous stems this growth goes on year after year, a layer of wood being annually added, and in the trees of temperate climates the number of these rings can be readily counted; the woody fibres first formed in the spring are larger and less dense than those formed later when vegetation is more sluggish, and this dif- ference in texture allows the line of demarca- tion to be seen, when there are not, as is some- times the case, ducts in the wood first formed. In most old stems the wood near the centre after a while becomes solidified by the incrus- tation of the cells, forming heart wood, which is essentially dead, as it takes no part in the growth of the tree ; the newer wood in which vegetation is still active is popularly known as sap wood. While the bark receives an annual layer upon its inner surface, the outer surface undergoes marked changes; it is acted upon by the elements, and soon loses its epidermis, often the corky layer, and eventually the only part left is the liber, which gradually dies on the exterior, and is renewed on the interior ; in some trees the bark falls away in flakes, in others in strips, the method of getting rid of the old bark varying greatly in different spe- cies. In a very old exogenous tree the only living portions are the buds which continue the growth and from which foliage is devel- oped, the fresh portions of the roots, and the new layers of wood and bark, with the in- tervening cambium which connects the roots with the buds ; examples of hollow trees are not rare in which, all the remaining portion of the stem has been removed by decay. The internal structure of the root is similar to that of the stem ; its chief increase is by lengthen- ing, and this takes place much more rapidly at the tip than elsewhere, or rather a very short distance back of the tip, the very point being a sort of cap or cushion of dead cells which serves to protect the living point as it pushes its way through the soil ; in some plants this root cap is very distinct. It was formerly sup- posed that the root was furnished with special absorbing organs, which were called spongi- oles, a term still sometimes used, though it was long ago shown that such organs have no ex- istence, and that the root absorbs anywhere through its recently formed parts. The root differs from the stem in branching irregularly and in usually bearing no buds, though under certain conditions it will, like other parts of the plant, form adventitious buds ; and many plants difficult to multiply otherwise are prop- agated by root cuttings, which when separated from the main plant will produce buds. Mo- nocotyledonom Stems. In the stem of monoco- tyledons a cross section shows no distinct ring of wood, and no separation into pith, wood, and bark, but bundles of woody fibre are irreg- ularly scattered all through the cellular tissue or pith, though less numerous in the centre than at the circumference of the stem ; these bundles have their origin in the leaves, and as new leaves are de- veloped new bundles push their way down through the central portion of the stem, in young plants reaching the roots, but in old- er ones finally curving outward and losing themselves in the rind. In these stems the newly formed wood is deposited within the older bundles, and they are called endogenous, or in- side-growing, endo- gen being synony- mous with monoco- tyledon. The inter- nal structure of a stem of this kind is shown in the article PALM ; and the corn stalk illustrates that of an annual endo- genous stem, which when young has an epidermis, but no separable bark. Each one of its fibrous bundles contains all the elements of the exogenous stem; a microscopic exami- nation of such a bun- FIG. 11. Section of a Mono- cotyledonous Stem (Indian Corn). FIG. 12. Bundle of Fibres from the above, containing the same parts as a dicotyle- donous stem. (See fig. 10.) dle shows bast cells, wood cells, ducts of various kinds, and a portion in which all the growth of the bundle is carried on, corresponding to