BOTANY. 15 



single terminal bud, as that just referred to, an example of which is to bo 

 found in tlio Palm, is called a p/ii/Uophor, or phyllogen. From this bud are 

 develope«! the leaves with which the vascular bundles are connected, forming, 

 as it were, their roots ; when the leaves of one bud decay another is produced 

 in the centre, the bases of the leaves, as they die and fiiU off, leaving a scar 

 on the stem. There is no way of determining the age of an endogen, a palm 

 for instance, by examining a cross-section, since there are no rings of growth ; 

 an approximation may, however, be found from the known length of the 

 tree, elongation proceeding pretty uniformly, and at a determinate rate for 

 different species. * Occasionally there are several terminal buds, Avhich 

 may cause a dichotomization or branching of the plant ; in many, however, 

 there is but a single one, whose decapitation is followed by the death of 

 the tree. 



The third kind of stem, the acrogenoiis, or acofi/ledonons, is, in general 

 appearance, not unlike that of most endogens, in being unbranched, of 

 nearly uniform diameter^ and bearing a tuft of leaves at the summit ; the 

 internal structure, however, always furnishes a ready means of distinction. 

 Acrogens are rarely arborescent ; a good illustration i^?, however, to be found 

 in the Tree Fern, the stem of which is called a rachis or stipe. A trans- 

 verse section exhibits a circle of vascular tissue, composed of masses of 

 various forms and size, near the circumference ; the centre is either hollow 

 or formed of cellular tissue. On the outside of the vascular circle there are 

 cells covered by a cellular integument, representing an epidermis, often very 

 compact, and formed originally of the bases of the leaves. The vascular 

 bundles are all formed simultaneously, and their number depends upon that 

 of the individual leaves or petioles. Although the acrogen is said to grow 

 only from the top, yet, strictly speaking, there is a slight increase in diameter, 

 as is shown in the separation of the rhomboidal leaf-scars, which originally 

 were in contact. These scars, or cicatrices^ are generally arranged in spiral 

 series around the stem, which always carries their traces. 



In thallogens, which are simple expansions of cellular substance, some- 

 times in definite directions, sometimes in all directions, there is no axis 

 whatever, nothing but threads woven together or separate, or else cells, lobes, 

 plates, or enlargements of various kinds. 



A few words as to the functions of the different parts of the stem must 

 conclude this portion of our subject. The office of the pith, as already 

 mentioned, is to convey nourishment to the young plant. By means of the 

 medullary sheath a connexion is kept up between the central parts of the 

 stem and the leaves, by means of spiral vessels, part of whose object may 

 be the transmission of air. The medullary rays preserve a communication 

 between the bark and the pith, and arc directly connected with the formation 

 of leaf buds and the matter of the cambium. The bark protects the tender 

 wood, conveys the elaborated sap doAvnwards from the leaves, and is the 

 uiedium in which many of the secretions are deposited. By means of the 

 vascular bundles the crude sap is conveyed from the roots to the leaves. In 

 woody fibre these bundles become ultimately choked up by the secondary 

 deposits. 



15' 



