84 
MORPHOLOGY OF TISSUES. 
the fascicular elements of the wood and of true bast ; but the fundamental tissue 
is also not unfrequently prosenchymatous. When, on the contrary, the cells are 
roundish or polyhedral, leave wide spaces between them, and touch one another by 
broad faces, the tissue is said to be parenchymatous. In elongated organs, like roots, 
internodes, &c., the cells are usually elongated in the direction of the axis of growth ; 
but are truncated at both ends, and arranged in parallel rows with broad septa. In 
green leaves, on the contrary, there are usually two kinds of parenchymatous tissue : 
— the so-called Palisade-pareyichyma beneath the upper epidermis, with cells elongated 
in a direction vertical to the surface of the leaf, but densely crowded together side 
by side ; and the Spongy Parenchyma of which the under half of the leaf consists, 
with roundish cells which either leave between them comparatively large intercellular 
spaces, or are furnished with outgrowths and branches with which they touch the 
adjoining cells, so that the tissue becomes still more spongy. The parenchyma attains 
the highest degree of looseness when the cells form a number of many-rayed stars 
which are in contact only by the ends of the rays, as in the stems of many rushes, 
the petiole of Musa, &c. The tissue of the larger Fungi (Fig. 55, p. 71) can be properly 
termed neither prosenchymatous nor parenchymatous ; but consists of a number of 
Hyphce, i. e. long slender branched filaments growing at the apex, and dividing trans- 
versely ; it is best to term it a Ryphal Tisstie. When the hyphae are densely interwoven, 
and their cells short and broad, a parenchymatous appearance is presented in transverse 
or longitudinal sections, and such a tissue has been called P seudo-parenchyma. 
It has already been mentioned that it is not unusual for individual cells in a tissue 
otherwise homogeneous to become developed in a manner strikingly different from 
their neighbours ; to such cells I have applied the term Idioblast. They may differ 
from the surrounding tissue in three different ways: — (i) Their form is the same, 
but they are distinguished by their contents ; as, for example, by a coloured cell-sap, 
or by containing a volatile oil, resin, gum, or other similar substance, in which case 
they are termed Simple Glands ; or they may contain groups of crystals, bundles 
of raphides, or cystoliths, in which case I have termed them Lithocysts. (2) In cells 
which contain a coloured sap, simple glands, and lithocysts, the cell-wall remains thin, 
while in other cases it becomes so thickened that the cavity of the cell is reduced 
to a narrow canal or a small central hollow ; the thickened cell-wall manifests strati- 
fication and pore-canals, and is usually very hard. Idioblasts of this kind may be 
included in the general term Stone-cells or Scleroblasts. They are seldom isolated, 
but more often associated in groups or layers, and then form the tissue already de- 
scribed as Sclerenchyma. (3) In the two kinds of idioblasts already described a 
tendency is exhibited to attain a larger size than that of the surrounding cells; 
scleroblasts are especially characterised by outgrowths. In the third group this takes 
place to a remarkable extent, this kind of idioblast being distinguished not only by 
its contents and the form of its cell-wall, but especially by its great increase in 
size and vigorous growth and branching. This is illustrated to a moderate extent in 
Fig. 16 (p. 21), more strikingly in the Spicular Cells of Gymnosperms (Fig. 52, p. 66), 
which usually contain a number of crystals in their thick wall, are considerable in 
size, and generally branch extensively. To this class belong also the internal 
'stellate hairs' of the petiole of Nuphar (Fig. 68, p. 81) ; and closely allied to these are 
the structures termed by Van Tieghem ^ ' poils ' in the fundamental tissue of the 
Monsterinese, a group of Aroideae. Fig. 70 represents a longitudinal section through 
the petiole of Monstera deliciosa ; a cell s lying in the middle parenchymatous row 
has branched right and left and put out two arms extending upwards and downwards 
into the intercellular spaces of the parenchyma, one of which has again put out a small 
^ Van Tieghem, Structure des Aroidees ; in Ann. des Sei. Nat. 1866, vol. VI. See also Otto 
Buch, Ueber Sclerenchymzellen. Breslau 1870. 
