128 



PHYSIOLOGICAL BOTANY. 



of a single cell the simplest possible form of vegetation (see 

 fig. 519). 



386. With a few such exceptions, vegetation consists of a 

 combination of cells united in a definite manner and form. Such 

 combinations are called tissues, which we may describe under 

 four general names or types : 



I. CELLULAR TISSUE (PARENCHYMA) : 

 II. FIBROUS TISSUE (PLEUREXCHYMA) : 



III. VASCULAR TISSUE (TRACIIEXCIIYMA) : 



IV. LATICIFEROUS TISSUE (CIENCHYMA). 



387. Parenchyma, composed of spheroidal cells, is the most 

 common 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 : 



1, when there are copious intercellular spaces, the cells slightly 

 touching, and being (a) rounded, or (b) lobed, or (c) stellate ; 



2, when the cells are crowded, leaving no intercellular space, and 

 being (d) prismatic, or (e) polyhedral, or (f) irregular. 



388. Examples of these tissues are found 

 (a) in the pulp of fruits, in newly-formed 

 pith, and in all young growths ; () in the 

 lower stratum of leaf-tissue ; (c) in the pith 

 of rushes and other aquatic plants ; (d) in 

 the herbaceous stems of Monocotyledons; 

 (e) everywhere, but well observed in full- 

 formed pith; (/) abundant in all the soft, 

 lleshy parts of plants. 



389. Pleurenchyma is 

 composed of elongated cells co- 

 hering 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 moder- 

 ate length, remarkable for its Medullary rays". 

 firmness, the main constituent of the stems and trunks of the 

 higher plants ; (b) liber, with very long attenuated cells, the sub- 

 stance of the inner layers of bark, remarkable for its tenacity, 

 especially in Flax, Hemp, Linden: 



491, Longitudinal section of Thuja (Red Cedar) o, 



