FUNCTIONS OF VESSELS. 23 



vessels convey air, which often contains an excess of oxygen in its 

 composition. Hales showed that air was evolved from the vessels 

 of the Vine when cut, and Decandolle thought that part of the air in 

 these vessels was derived from the pores of the leaves. Hofiman 

 from his experiments concludes that spiral vessels in the ordinary 

 state contain air, but that when a large quantity of fluid is applied 

 to the leaves it enters the spirals. Other authors look upon these 

 vessels as conveying fluids, while a third set maintain that both air and 

 fluids are present, the air being derived in part from decompositions 

 going on in the interior of the plant. The other kinds of vascular 

 tissue, and especially the pitted vessels, are the means by which the 

 fluids taken up by the roots of plants are conveyed to the leaves, and 

 to all parts of the plants. Laticiferous vessels contain, according to 

 Schultz, the elaborated sap or latex on its return from the leaves to 

 the bark. This latex is either transparent or opaque, colourless or 

 coloured. These vessels, when examined with the microscope in the 

 living plant, exhibit movements in their fluid contents of a peculiar 

 kind, which will be considered under Cydosis. 



The cell has been already shown to be the type of all the tissues of 

 plants, and to be the basis of aU vegetable structure. It is of equal im- 

 portance as regards function. In the lowest plants, as the PalmeUa 

 (Protococcus) nivalis, or the Alga found in red snow, and other species of 

 FalmeUa (fig. 44), also in Nostoc and Hsematococcus, cells constitute 

 the whole substance, and perform all the functions of life ; they absorb 

 and assimilate, thus performing the functions of nutrition and secretion, 

 and they form new cells, thus reproducing individuals like them- 

 selves. When a more complex structure exists, as in the higher tribes 

 of plants, certain cells are appropriated for absorption, others are con- 

 cerned in assimilation, and others in forming and receiving secretions. 

 When a certain degree of solidity is required to support the stem, 

 leaves, and flowers, ligneous matter is deposited, and bast fibres 

 are formed. When the transmission of fluids and air is carried on 

 rapidly, the elastic fibres of the fibro-vascular tissue seem to keep the 

 elongated ceUs and vessels pervious, and when the elaborated sap is 

 conveyed continuously without interruption, anastomosing tubes occur 

 in the form of laticiferous vessels. Cells and vessels are thus difiier- 

 entiated for the performance of special functions. 



Tabulae Abbangement of Veqetablb Tissues. 



A. — Cellular Tissue (Parenchyma), composed of membrane, or of membrane and 

 fibre, having the form of vesicles whose length does not greatly exceed 

 their breadth. 

 1. Membranous Cellular Tissue; cells formed by membrane alone, of varying 

 thickness, but without markings on it ; when thickened and fusiform 

 they constitute prosenchyma, composed of bast cells. 



