THE TISSUES. 



186 



Such combinations are called tissues, -wWcli we may describe under four 



general names or types : 



I. Cellular tissue (Parenchyma) : 

 n. Fibrous tissue (Pleukenchyma) : 



III. Vascular tissue (Trachenohyma) : 



IV. Latioiferous tissue (Cienchyma). 



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

 jnon 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 (S) lobed, or (c) 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 are found (a) in the pulp of fruits, in newly-formed 

 pith, and in all young growths ; (6) 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) everywhere, but well observed in full-formed pith ; (/) abundant in all 

 the soft, fleshy parts of plants. 



C 666. Pleurenchyma is composed of elongated cells cohering by their 



sides in such a way that end overreaches end, forming a continuous ^fij-e. 



Two varieties are noticed (a) wood- 



fihre, with cells of moderate length, 



remarkable for its firmness, the 



main constituent of the stems 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, 



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

 (intercellular cavity right opposite outside. (470). 



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



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



cells joined end to end, and fused into one by the absorption of the 



579, Longitudinal section of Thuja (Red Cedar). 

 u, Medullary rays. 



