THE TISSUES. 



135 



Such combinations are called tissues, which we may describe trader four 

 general names or types : 



I. Cellular tissue (Parenchyma) : 



II. Fibrous tissue (Pleurenchyma) : 



III. Vascular tissue (Trachenchyma) : 



IV. Laticiferous tissue (Cienchyma). 



664. Parenchyma, composed of spheroidal cells, is the most com- 

 mon form of tissue, no plant being without it, and many, especially of 

 the lower orders, being entirely composed of it. Numerous varieties 

 occur according to the forms of the cells and their closeness of contact, 

 intermediate between the following extremes, 1, when there are copious 

 intercellular spaces, the cells slightly touching, and being (a) rounded, 

 or (b) lobed, or (o) stellate ; 2, when the cells are crowded, leaving no 

 intercellular space and being (d) prismatic, or (e) polyhedral, or (/) ir- 

 regular. 



665. Examples of these tissues arc found (a) in the pulp of fruits, in newly-formed 

 pith, and in all young growths ; (b) in the lower stratum of leaf-tissue ; (c) in the 

 pith of rushes and other aquatic plants; (d) in the herbaceous stems of Monocoty- 

 ledons ; (e) every where, but well observed in full-formed pith ; (/) abundant in all 

 the soft, fleshy parts of plants. 



666. Pleurenchyma is composed of elongated cells cohering by their 

 sides in such a way that end overreaches end, forming a continuous fibre. 

 Two varieties are noticed (a) wood- 



fibre, with cells of moderate length, 

 remarkable for its firmness, the 

 main constituent of the steins and 

 trunks of the higher plants ; (6) 

 liber, with very long attenuated 

 cells, the substance of the inner 

 layers of bark, remarkable for its 

 tenacity, especially in flax, hemp, 

 linden. 



667. The pitted cells (§ 650) 

 constitute a singular variety of 

 wood-fiber, common in pines, firs, 

 etc. That mysterious double ring 

 which encircles each pit, is pro- 

 jected, the inner by the pit itself, 



579, Longitadin.il section of Thuj:i(RecI Cedar). 

 a, Medullary rays. 



which is an aperture in the secondary layer, the outer by a lens-shaped 

 intercellular cavity right opposite outside. (570). 



668. Trachenchyma is a tissue of vessels or tubes rather than cells. 

 The vessels are extended lengthwise, and composed each of a row of 

 cell* joined end to end, and fused into one by the absorption of th« 



