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, 'VIZ. 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 ^/oe, it 
forms only the superficial layers, while the inner mass of tissue consists of colourless 
parenchyma. 
The hypodermal layers, bundle-sheaths, and intermediate 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 
Ni4phar, in the root of Pihilaria, in the petiole and stem of Monsterinea;, &c., or finally, 
the laticiferous cells of Euphorbiacese, 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 Alisma 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 aquilina 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 
^ Nägeli, Ueber das Wachsthum des Stammes u. der Wurzel, in Beiträge zur wiss. Bot. Leipzig 
1858, Heft I. — Sanio, Bot. Zeitg. 1865, P- 165, et seq. — Millardet, Sur l'anatomie et le developpement 
du corps ligneux dans les genres Yucca et Braccena, in Mem. de la Societe Imper. des Sei. Nat. de 
Cherbourg, vol. XI, 1865. — On abnormal formations of wood in Dicotyledons see Crüger, Bot. 
Zeitg. 1850 and 1851. — Nägeli, 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, 
Recherclies sur la symetrie, &c., Ann. des Sei. Nat., 5th ser., vol. XIII. 
