CHAMBERS'S INFORMATION FOR THE PEOPLE. 



algae are covered with cilia, which give them loco- 

 motive power ; the cells of hairs are elongated, 

 and often much branched ; and the cells of unicel- 

 lular algae, such as the Desmideae and Diatomaceae, 

 often display the most exquisite symmetry, both 

 in outline and in the elaborate sculpture of their 

 surface. The cotton hair, which is a cell in 

 peculiar condition, presents the form of a flat 

 twisted band with a thickened margin, a character 

 which is retained by it after it is dressed, spun, 

 and woven into cloth, and even after that cloth 

 has been worn to rags, reduced to pulp, and 

 remanufactured into paper. 



In the pulp of leaves and fruit, and in the 

 cellular tissue of the bark, there are frequently 

 cavities found among the cells, which are of several 

 kinds. Those called receptacles of secretion are 

 formed for the reception of the oils and other 

 fluids secreted by plants ; as, for example, the 

 fragrant oil in the myrtle and the orange, and the 

 turpentine in the pine and fir tribe. Other similar 

 cavities, called air-cells, contain oxygen nearly in 

 a pure state, and are called intercellular spaces. 

 All these cavities have no distinct membrane to 

 inclose them, but are surrounded by what may be 

 called a wall of cells, which form part of the 

 cellular tissue. If the stem of the common Hip- 

 puris, or mare's-tail, or the leaf-stalk of a water- 

 lily, be cut across, a beautiful arrangement of 

 these intercellular spaces will be displayed, and 

 the manner in which they are formed by the 

 peculiar arrangement of the plant's cells is shewn 

 in wood-cut b (page 69). 



Cellular tissue readily decays when the parts 

 composed of it fall from the plant. In leaves, the 

 pulpy parts disappear first, leaving behind the outer 

 cuticle and the nerves or veins, which are of firmer 

 texture ; the latter, indeed, being composed prin- 

 cipally of woody tissue, the tubes of which have 

 been filled with earthy matter during the process 

 of vegetation, decay very slowly. Those parts of 

 a plant which nature seems to have intended not 

 to be of long duration such as the fleshy parts of 

 the leaves, the flowers, and the fruit are composed 

 entirely of cellular tissue of loose texture. 



Woody Tissue (Pletirenchymd). The stems of 

 trees and of flowering-plants in general possess a 

 tenacity not found in the leaves and flowers. This 

 is mainly due to the presence of woody tissue, 

 which consists of minute spindle-shaped tubes 

 lying closely together, and overlapping each other 

 at the ends. The strength of these tubes is mainly 

 due to the deposit of ligneous matter on their 

 inner surface. The value of many plants employed 

 in the arts depends upon the abundance of this 

 tissue : when separated from the softer tissues of 

 the stem and leaves by maceration, it forms the 

 fibre of flax, hemp, jute, Chinese grass, and other 

 textile substances well known in commerce. In 

 cone-bearing plants, such as the Scotch fir, the 

 woody tissue is very peculiar, each tube exhibiting 

 a series of round discs with a central dot. By 

 carefully studying the peculiarities in such tissues, 

 the sources of unknown timbers may often be 

 determined. In most pines there is a single row 

 of discs on each tube, while in others a double 

 row of opposite discs is observed. In the Arau- 

 carias of the southern hemisphere, the discs are 

 angular, and are arranged alternately in several 

 rows ; and in the yew, the tube has beautiful 

 spiral markings as well as minute discs. Such 



70 



characters are invaluable to the student of veget- 

 able palaeontology. 



Vascular tissue has been divided by modern 

 botanists into three kinds namely, vascular 

 proper, pitted, and latictferous. Vascular tissue 

 consists of cylindrical tubes of great delicacy and 

 thinness, called spiral -vessels and ducts. Spiral 

 vessels are so called because they contain delicate 

 fibres coiled round in a spiral manner. They are 

 of a light elastic nature, and their fibres, though 

 coiled up naturally like a cork-screw (see fig.), 

 may be unrolled to a considerable extent. If a 

 leaf-stalk of a geranium or strawberry be cut half 

 through, and then doubled down first on one side, 



Spiral Vessel. 



and then on the other, and the two pieces be then 

 carefully and gently drawn asunder, the trans- 

 parent membrane will break, and the spirals will 

 unroll so as to appear, when seen with the naked 

 eye, like fine hairs between the two portions of 

 the leaf-stalk. Spiral vessels prevail in leaves 

 and flowers, and are found, though more sparingly, 

 in the young greenwood of trees and shrubs ; 

 but rarely in the old solid wood, roots, or bark. 

 They are few in coniferous trees ; but abundant 

 in palms and their allies. In ferns and the club- 

 mosses they occur occasionally, but are usually 

 replaced by a peculiar kind of vascular tissue, 

 called the scalariform tissue, from the ladder-like 

 appearance on the angular vessels, caused by bars 

 of fibre in their interior ; the other cryptogamous 

 or flowerless plants have no vascular tissue. 



Pitted tissue, sometimes called dotted ducts, 

 consists of tubes which, when viewed by trans- 

 mitted light, appear full of holes. This depends 

 on the incrusting matter or cellulose inside being 

 unequally deposited over the surface of the mem- 

 brane, and thus leaving uncovered spots at various 

 intervals. The dotted ducts are larger than the 

 vessels of the other tissues. They frequently 

 exhibit contractions at intervals, which give them 

 a jointed or bead-like appearance. In such 

 cases, they seem to be formed of dotted cells 

 placed end to end, with the partitions between 

 them obliterated, so as to form continuous cylin- 

 drical tubes. Laticiferous tissue consists of tubes 

 which are distinguished from all other kinds 

 of tissue by being branched. They are filled 

 with a fluid called latex, of a granular nature, 

 often milky or coloured, and exhibiting move- 

 ments. This milky fluid abounds in the India- 

 rubber and gutta-percha plants, and is seen 

 to exude in abundance from the dandelion when 

 its leaves or stalks are wounded. The fluid, in 

 many plants, contains a large quantity of caou- 

 tchouc ; it is bland and nutritious in the cow-tree 

 of the Caracas, but in many other plants narcotic 

 and acrid. 



Multifarious as are the modifications of the 

 tissues of plants, and the cells and vessels of which 

 they are composed, all have a common origin, all 

 are modifications of the cell. The plant begins its 

 existence as a cell, and its whole course of develop- 

 ment may be said to be the evolution of cells. The 

 process of cell-development, or cytogenesis, thus 

 explains the whole phenomena of plant growth and 

 reproduction. To the investigation of this subject, 





