454 BOTANY. 



leaves, and in the roots. In these the sap is filtered before 

 passing into the larger vessels. 2. The Porous^ or those which 

 are transversely furnished with pores ranged in lines. These 

 are observed throughout the whole vegetable ; but they are 

 not continuous, and terminate in cellular tissue. In compact 

 wood the pores are excessively fine. 3. The Spurious Tra- 

 chece, or tubes split transversely, differing from the preceding 

 only by these clefts, and chiefly remarked in porous wood. 

 These are the principal canals for the sap, which by means 

 of the clefts is enabled to spread laterally 4. The trachece are 

 composed of silvery elastic laminae in a spiral or double spiral 

 form. They surround the pith or central soft part of Dicoty- 

 ledonous vegetables, and are concentrated towards the woody 

 fibres of monocotyledonous stems. They are never found in 

 the annual ring or bark, and but rarely in the roots ; but they 

 abound in the spongy tissues of vegetables which grow rapidly. 

 5. The mixed vessels are composed of the four previous descrip- 

 tions modified and transformed into one another in their course. 

 With the exception of the tracheal vessels the others are bent 

 on all sides, and degenerate at their extremities into cellular tis- 

 sue. 6. The proper vessels, which are neither porous nor have 

 any opening in their walls, contain the peculiar fluids of the plant. 

 They are common to the bark, to the leaves, petals, &c. as well 

 as the trunk. Some are in fasciculi ; others solitary. Vegetables 

 are besides covered with an epidermis formed of dried plates of the 

 cellular tissue. In some, as the cork, this epidermis is composed 

 of many thick strata or layers. 



The more imperfect vegetables, or the Acotyledonous plants, 

 as the Fungi, Lichenes, Algte, and Tremellce, are simply cellu- 

 lar. The Monocotyledonous vegetables have, besides this cel- 

 lular tissue, porous and tracheal vessels ; and in the Dicotyledo- 

 nous plants all the kinds of tissue are combined. 



It was formerly believed that vegetables were nourished al- 

 most by water ; but it is now known that the water is decompos- 

 ed in the tissue of plants, to which it affords hydrogen ; and that 

 this water is loaded also with a number of substances of vegetable 

 and animal extraction. The carbonic acid in the air, and other 

 substances absorbed by the leaves, also furnishes a supply of 

 food to plants. 



The sap or juice of plants is a colourless and transparent fluid, 

 composed of water, of a mucilaginous extractive matter, sa- 



