54 THE ELEMENTARY STRUCTURE OF PLANTS. 



Filro-vascular Tissues, or the FIBRO-VASCULAR SYSTEM, as distin- 

 guished from the Horizontal, PARENCHYMATOUS, or common CEL- 

 LULAR SYSTEM of the plant. 



65. Intercellular System, The only exception, if such it be, to 

 the statement that all the vegetable tissues are formed of cells, is 

 that of the so-called vessels of the latex, which, according to the 

 view now best supported (63), do not so originate, but are a secon- 

 dary formation, resulting from the transudation of peculiar assimi- 

 lated matters into the interspaces between the cells ; and are there- 

 fore rather to be classed with other receptacles, canals, or inter- 

 vals that are found among or between the cells. Some of these 

 are due to imperfect contact or cohesion, and are in some sort 

 accidental, or at least are irregular and indefinite : such are 

 the INTERCELLULAR SPACES or PASSAGES, left when the angles in 

 parenchyma do not accurately fit throughout. Such are the 

 larger and irregular winding passages in the looser tissues called 

 merenchyma (51), as in the lower stratum of the leaf (Fig. 7), or 

 those formed by the lobed or branching shape of the cells them- 

 selves, so disposed as to join each other only by their extremities, 

 as is seen in many water-plants. These spaces are soon filled 

 with air. There are besides, in the stems and foliage of aquatic 

 and marsh plants, an abundance of much larger AIR-CELLS or 

 AIR-PASSAGES, usually of many times greater diameter than the 

 cells of the tissue, and produced by their particular arrangement. 

 These are as elaborately built up as any proper organ can be, are 

 constructed upon a uniform plan in each species, and are evidently 

 essential to its existence, such plants requiring a full supply of air 

 in their interior. Other air-spaces or empty intervals, apparently 

 less essential to the life of the plant, arise from the destruction of 

 a part of the parenchyma, either by absorption, or by distention, 

 from the more rapid enlargement of the outer part. In this way, 

 the stem or the pith of many plants becomes hollow. 



66. Receptacles of Special Secretions, These arise from the ex- 

 udation of the proper juices of the cells into the intercellular pas- 

 sages, which are distended by the accumulation; and often the 

 contiguous cells are destroyed, so as to form cavities of considera- 

 ble size. Such are the turpentine canals of the Pines, &c. ; the 

 oil-cells of the fruit of the UmbelliferEe, and in the rind of the 

 orange and lemon ; the latex-canals in Sumach, &c. 



67. Internal Glands, such as those which form the translucent 



