508 PHYSIOLOGY. 



characterized by peculiar modes of combination of the cells, viz. felted 

 tissue (tela contexta) and vascular tissue. 



Imperfect parenchyma (merenchyma) is composed of cells with more 

 or less rounded surfaces connected into a lax tissue, necessarily presenting 

 abundant intercellular passages and spaces. The cells are tolerably uni- 

 form globular or oval (a), or lobed, and connected at few points, leaving 

 wide intercellular passages between them (b) ; in other cases the cells are 

 more or less stellate, and leave large spaces between them (c). 



The form a is common in all young organs of the higher plants, espe- 

 cially in the rind and the pith (fig. 520), in the p ; ilp of fruits, &c. ; b is 

 very characteristic of the lower stratum of the internal substance of leaves 

 (fig. 522) ; c occurs in the stems and leaf-stalks of aquatic plants, in the 

 pith of Rushes (fig. 524), &c. 



Perfect parenchyma is composed of cells bounded and united together 

 by plane surfaces ; where the cells are regular polyhedra, of about equal 

 size, the tissue is (a) regular parenchyma ; if the size is unequal and the 

 forms unlike, the tissue becomes (b) irregular parenchyma. Certain modi- 

 fications of regular parenchyma have received distinct names, viz.: (c) 

 prismatic parenchyma, where the cells are 6-sided prisms with pyramidal 

 ends ; (d) muriform parenchyma, where the cells are square or oblong, 

 with the long diameter horizontal, and packed like bricks in a wall ; and 

 (e) tabular parenchyma, where the cells are flattened from above down- 

 wards. 



The form a is abundant throughout all classes of plants, and is well 

 seen in fully developed pith of Dicotyledons (fig. 526) ; b is even more 

 common in the soft parts of plants (fig. 527) ; c is met with in the her- 

 baceous stems of Monocotyledons, and in the upper part of the diachyma 

 of leaves, also in a woody condition in the testa of various seeds ; d is 

 characteristic of cortical structures, and may be seen in cork, peri derm 

 of Birch, the rind of the rhizome of Tamus &c., also in the medullary 

 rays of Dicotyledons ; e occurs specially in the epidermal cells. 



Merenchyma and parenchyma in their various modifications run into 

 one another by countless intermediate conditions. 



Sclerenchyma consists of ordinary cellular tissue, the constituent 

 cells of which become ultimately filled with stratified woody thick- 

 enings. They occur locally, even as individual cells or in groups 

 (sderites\ or even in more or less continuous layers. A familiar 

 illustration occurs in the "grit" of Pears. Their purport is sup- 

 posed to be to protect and support softer tissues. 



Prosenchyma is composed of cells elongated greatly in one di- 

 rection, and attenuated to a more or less acute point at each end, 

 forming what is called a fibre. These fibres are necessarily united 

 for the most part by their lateral surfaces, and their ends are insi- 

 nuated into the spaces between those lying above and below them. 



"We distinguish in prosenchyma two modifications (a) woody 

 fibre, composed of spindle-shaped cells of moderate length, and (b) 

 liber or bast cells, composed of very long slender cells which are 

 occasionally slightly branched. 



