INTERNAL ANATOMY OF ORGANS. STEMS. 525 



without any proper lining- wall, but surrounded by thin -walled 

 cells tilled with resin and other secretions more or less devoid of 

 oxygen, and which are poured into the cavity lying in the midst 

 of the parenchyma, or in the liber and wood, parallel with the 

 fibro-vascular structures. Similar reservoirs exist in the roots of 

 Rhubarb, in the leaves of Aloes., &c., in varying positions. The 

 cells which bound the cavities sometimes grow and project into them, 

 more or less filling them up. 



Sect. 4. INTERNAL ANATOMY OF ORGANS. 



All young plants are composed of cellular tissue alone ; and the 

 Thallophytes never acquire any of the more highly developed 

 " systems " which we meet with in full-grown Flowering Plants 

 and the higher Cryptogamia. In the stems of the latter, the 

 " systems " present special modes of arrangement, respectively 

 characteristic of the great Classes. In embryo plants the tissues 

 have, according to Hanstein, a three-fold origin in dermatogea, 

 periblem, and plerome (see ante, p. 515); and these three layers 

 are distinguishable before even the formation of the cotyledons. 

 Famintzin considers them identical with the embryonic layers of 

 the animal. 



The more or less uniform condition of the tissues in the Thallophytes is 

 connected with great simplicity in the physiological processes of vegeta- 

 tion and growth ; while in the higher plants the difference of internal 

 organization is accompanied by important differences in the modes of 

 development of the axis. It would cause us to exceed our limits very 

 widely to enter into minute details of the internal structure of the organs 

 of vegetation of plants generally ; but it is requisite not only to give a 

 general sketch of the plan of organization, but to describe some of the 

 more important modifications met with in the higher Classes. 



Structure of Stems. As a general rule, plants possessing stems 

 and leaves exhibit in their stems a definitely arranged fibro-vas- 

 cular system, the bundles of which send off branches, or pass off 

 themselves entirely, to form the ribs and veins of the leaves. The 

 young stem is made up of wood-cells and vessels, placed the one 

 within the other, superposed in rays and surrounded by connecting 

 cellular tissue. The same axial system furnishes below, directly 

 or indirectly, the bundles which constitute the woody central mass 

 of roots, in which originally the liber and the vessels are placed 

 side by side. 



Mosses. The simplest form of the fibro-vascular system is seen in the 

 Mosses (p. 431), where a cord of prosenchymatous tissue runs up the centre 

 of the thread-like stern, and in some cases sends off branches to the leaves. 



Lycopodiaceee. In the Lycopodiacese (p. 423) the axis of the stem is 

 occupied by one or more parallel fibro-vascular bundles, containing spiral 



