FUNCTIONS OF VESSELS. 21 



scope in the living plant, exhibit movements in their fluid contents of 

 a peculiar kind, which will be considered under Cyclosis. 



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

 of plants, and to be the basis of all vegetable structure. It is of equal 

 importance as regards function. In the lowest plants, as the Protococ- 

 cus nivalis, or the Alga found in red snow, and the various species of 

 Palmella, Nostoc, and Hsematococcus, cells constitute the whole sub- 

 stance, and perform all the functions of life ; they absorb and assimi- 

 late, thus performing the functions of nutrition and secretion, and they 

 form new cells, thus reproducing individuals like themselves. When 

 a more complex structure exists, as in the higher tribes of plants, 

 certain cells are appropriated for absorption, others are concerned in 

 assimilation, and others in forming and receiving secretions. When a 

 certain degree of solidity appears to be required to support the stem, 

 leaves, and flowers, ligneous matter is deposited, and woody fibre 

 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 

 cells and vessels pervious, and when the elaborated sap is conveyed 

 continuously without interruption, anastomosing tubes occur in the 

 form of laticiferous vessels. 



TABULAR ARRANGEMENT OF VEGETABLE TISSUES. 



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

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

 ceed their breadth. 



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



thickness, but without markings on it. 



2. Porous Cellular Tissue; cells formed by membrane, which has been une- 



qually thickened in such a way as to leave rounded depressions at 

 regular intervals. 



3. Fibrous Cellular Tissue (Inenchyma); cells formed by membrane and fibre; 



occasionally formed by fibre alone. 



a. Spiral Cells, with a complete spiral fibre inside. 



b. Dotted Cells, with opaque spots, which are isolated portions of fibre. 



B. Vascular or Tubular Tissue (Angienchyma), composed of cylindrical tubes, 

 which are more or less continuous, and usually overlap each other, 

 or are united by broad oblique extremities. 



I. Membranous Vascular Tissue; tubes formed by membrane alone, of vari- 



ous thickness, but without markings on it. 



1. Ligneous Tissue (Pleurenchyma), composed of fusiform tubes with 



thickened walls. 



2. Laticiferous Tissue (Cinenchyma), composed of tubes which anastomose, 



often present irregular dilatations, and convey a peculiar fluid, called 

 Latex. 



II. Porous Vascular Tissue; tubes formed by membrane, which becomes 



thickened by spiral deposits, in such a way as to leave rounded de- 

 pressions at regular intervals. 



1. Vasiform Tissue, or Porous vessels (Bothrenchyma or Taphrenchyma); 

 large tubes, usually ending in broad extremities, with pits or circular 

 markings on their walls. This tissue sometimes exhibits contractions 



