410 THE LEAVES. PART n. 



are of cellular tissue enclosed in vascular tubes and 

 spiral vessels, which, terminate at a little distance from 

 the extremity, leaving a point of loose spongy cellular 

 tissue, called the spongiole, which absorbs from the 

 ground the liquids that nourish the plant. These root 

 fibrils are temporary organs; they die on the older 

 parts of the subterranean branches, and are succeeded 

 by others on the new. 



The various tissues which form the stem of a tree 

 form, in the same manner, though in diminished num- 

 bers, the complicated ramifications of the branches and 

 the leaf-stalks, and terminate in the leaves themselves. 

 Under the transparent film which forms the skin on the 

 upper- surface of a leaf, there is a layer of soft thin- 

 walled cylindrical or prismatic cells, closely pressed to- 

 gether, and full of green vegetable matter, or chloro- 

 phyll. Several layers of thick- walled cells follow, each 

 more loosely aggregated than that which precedes it, 

 and fuller of void spaces, till in the last green layer on 

 the under-side of the leaf the cells are globular, with 

 numerous large irregular void spaces, united in a reti- 

 culated system filled with air, and in direct communi- 

 cation with the atmosphere by means of the innumerable 

 stomata, which are to be found in the under-surface of 

 the leaves of all land plants of the higher classes, and 

 which are their organs of respiration. 



The form of the leaf is determined by the arrangement 

 of the vascular bundles, which are in communication 

 with those in the interior of the stem, and branch out 

 in various directions through the green layers : these 

 branches unite again, and form the skeleton of the leaf, 

 which is often a delicate maze of the finest lacework of 

 nerves. The vascular system is double, consisting of 

 an ascending and descending portion. The ascending 

 portion, which is continuous with the medullary sheath, 

 becomes continuous at the apex of each nerve of the 



