THE FUNDAMENTAL TISSUE. 125 



When it is parenchymatous, it may be termed simply Fundamental Parenchyma. Two 

 principal forms of this may be distinguished, which are nevertheless united by transi- 

 tional forms, "z-'/'x. the colourless parenchyma which occurs in the interior of large 

 succulent stems and tubers, and in all roots and succulent fruits, and the parenchyma 

 containing chlorophyll which forms the superficial layers beneath the epidermal tissues 

 of stems and fruits. In foliage-leaves, when thin and delicate, it fills up the space 

 between the upper and lower epidermis ; if they are very thick, as in Aloe, it 

 forms only the superficial layers, while the inner mass of tissue consists of colourless 

 parenchyma. 



The hypodeimal layers, bundle-sheaths, and interm.ediate tissue are the ordinary 

 and essential constituents of the fundamental system ; but in addition forms of cells 

 and tissues developed in a peculiar manner occur, and much more frequently than in 

 the fibro-vascular bundles. Of the former kind are the majority of the cells described 

 as idioblasts in Sect. 14, isolated cells containing a pigment, tannin, a volatile oil, clusters 

 of crystals, &c., or large utricular vessels, or isolated scleroblasts, or the branched cells 

 comprised under the term trichoblasts, such as spicular cells, the hairs in the interior of 

 Nuphar, in the root of Pilularia, in the petiole and stem of Monsterineae, &c., or finally, 

 the laticiferous cells of Euphorbiaceae, Moreae, Asclepiadeae, and Apocynaceae. True 

 laticiferous vessels are, on the other hand, less often found in the fundamental tissue ; 

 but in the cortex of many Liliaceae they are replaced by utricular vessels (see 

 Sect. 14). Among the more complicated forms of tissue which occasionally enter into 

 the composition of the fundamental tissue may be named true (compound) glands, or 

 more frequently secretion-canals containing gum, resin, a volatile oil, or even latex, as 

 in AUsma and Rhus. Of very common occurrence are, moreover, groups or layers of 

 scleroblasts (especially in the cortex of many woody plants and the juicy flesh of pears), 

 and layers, bundles, or bands of brown-walled sclerenchyma (in Pteris aquil'ina and Tree- 

 ferns). Attention has already been called to the sclerenchyma of which the stone of 

 stone-fruit (drupes) consists as a form of fundamental tissue ; the natural contrast to this 

 is the pulp or flesh of berries and of many stone-fruits. 



Sect. 18. The Secondary Increase in Thickness of Stems and Roots ^ — 



During the period when the younger portions of stems and branches are still 

 increasing in length, they are also increasing in girth, the primary meristem 

 becoming differentiated into other tissues which grow not only in the direction 

 parallel to the axis of growth, but also in the radial and tangential directions. 

 At an early period roots attain, immediately behind the growing point, the size which 

 they retain^until they have ceased growing in length. 



This increase in diameter of stems, which accompanies, or even for a 

 short time outlasts, the growth in length, is frequently occasioned mainly by the 

 tangential extension of the outer layers of tissue, while that of the pith does not 

 keep pace with it. The pith will then split and the stem become hollow ; and 

 this is often carried to such an extent that the substance of the cylinder itself 



^ Nageli, Ueber das Wachsthum des Stammes u der Wurzel, in Beitrage zur wiss. Bot. Leipzig 

 1858, Heft I. — Sanio, Bot. Zeitg. 1865, p. 165, etseq. — Millardet, Sur I'anatomie et le developpement 

 du corps ligneux dans les genres Yticca et Dracazna, in Mem. de la Societe Imper. des Sci. Nat. de 

 Cherbourg, vol. XI, 1865. — On abnormal formations of wood in Dicotyledons see Criiger, Bot. 

 Zeitg. 1850 and 1851. — Nageli, Dickenwachsthum des Stengels u. s. w. bei den Sapindaceen. Munich 

 1864. — Eichler, Ueber Menispermaceen, in Denkschrift der k. bayer. bot. Gesellschaft zu Regensburg, 

 1864, vol. V. — Sanio, Bot. Zeitg. 1864, p. 193 et seq. — Askenasy, Botanische morphologische 

 Studien, Dissertation, Frankfort-a-M. 1872.— On the increase in thickness of roots see Van Tieghem, 

 Recherches sur la syinetrie, &c., Ann. des Sci. Nat,, 5th ser., vol. XIII. 



