120 BOTANY part i 



In t.01110 of the Piperaceae aud Bcgoniaceae, and iu some species of Ficus, the 

 epidermis is composed of several layers ; but this is a comparatively rare occurrence. 

 Such a many-layered epidermis results fi'om a division of the young epidermal 

 cells parallel to the external surface. The epidermis of Ficus elastica (Fig. S3) 

 has three layers, and serves as a reservoir for accumulating water. The cystoliths 

 of Ficus elastica, already referred to (p. 76), occur in single swollen epidermal cells. 

 A many-layered root epidermis is also met with, as in species of Asjmragiis, Crinum, 

 and Lycoris. The many-laj'ered epidermis of the aerial roots of many Orchids, 

 and of various Aroids, undergoes a peculiar modification and forms the so-called 

 VELAMEN KADICUM (p. 49), a parchment-like sheath surrounding the roots, and 

 ofteu attaining a considerable thickness. The cells of this enveloping sheath are 

 generally provided with spiral or reticulate thickenings, aud lose their living 

 contents. They then become filled Avith either water or aii", depending upon the 

 amount of moisture contained in the surrounding atmosphere. These root-envelopes 

 absorb water like blotting-paper ; when the velamen is filled with water the under- 

 lying tissues impart a greenish tint to the root ; but if it contains only air the 

 root appears white. The epidermis of fruits, and iiarticularly of seeds, exhibits a 

 considerable variety of modifications in its mode of thickening, and in the relations 

 the thickening layers bear to one another. The purpose of these modifications in 

 the epidermis becomes at once evident, when it is taken into consideration that, 

 in addition to protecting and enclosing the internal parts, the epidermis has often 

 to provide for the dissemination and permanent lodgment of the fruits and 

 seeds. 



The Vascular Bundle System. — The primary vascular bundles 

 extend in the form of strands throughout the body of the higher 

 plants. In more transparent stems, such as those of Impatiens parvi- 

 flora, the bundles may be clearly distinguished aud their course 

 followed. The arrangement of the bundles of leaves is apparent from 

 the venation. In many parallel-veined leaves the bundles are easily 

 isolated. This is often done accidentally, as "when, for example, in 

 picking a leaf of Plantain (Plantago media) a pull is given at the same 

 time. 



Special strands of tissue serving for the transport of substances 

 througli the plant are found in the more highly difterentiatcd Thallo- 

 phyta ; examples ai'e afforded by some of the Ked and Brown Seaweeds 

 (Rhodophyceae and Pliaeophyceae). In the Laminariaceae these con- 

 ducting tracts contain elements which closely resemble sieve-tubes 

 i}-^). The thallus of some Liverworts is traversed by a strand which 

 resembles the nerve of a leaf. Bundles sharjily limited from the 

 surrounding tissties first aj^pear in the Mosses ; they occiu' commonly 

 in the leaves, less often in the stems. A fairly simple example of 

 this kind of conducting bundle is that of the stem of Mninm nndidatum 

 which is represented in transvei'se section in Fig. 172. Such bundles 

 reach their highest differentiation in the Polytrichaceae. In them 

 the stem has a central cylinder composed of elongated cells with 

 scanty contents, of elements resembling sieve-tubes, and of elongated 

 cells; the three kinds of element serve respectively to transport 

 water, albuminous substances, and carbohydrates {^-~). The arrange- 



