84 MORPHOLOGY OF TISSUES. 



the fascicular^elcments 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-parenchyma 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 M'hich 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- 70 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 Hyphal Tissue. 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 Pseudo-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-pells 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 oi Nuphar (Fig. 68, p. 81); and closely allied to these are 

 the structures termed by Van Tieghem^ 'polls' in the fundamental tissue of the 

 Monsterineae, 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 Sci. Nat. 1866, vol. VI. See also Otto 

 Buch, Ueber Sclerenchymzellen. Breslau 1870. 



