THE TISSUES OF THE STEM 53 



embedded in thin-walled conjunctive parenchyma of the stele, and is 

 surrounded by an indefinite sheath of sclerotic fibres. It consists of 

 xylem directed as in the Dicotyledons to the centre of the stem, and 

 phloem towards the periphery. The xylem is represented by two or 

 three annular or spiral vessels of the protoxylem (a, sp.) } adjoining 

 a large air-space (/), and two large pitted vessels (m, m'), with a bridge 

 of fibrous tracheides coupling them together. The number of these 

 vessels may vary, especially near to the nodes. Together they form 

 a V-shaped group, and the phloem is fitted between the limbs of 

 the V. It consists entirely of sieve-tubes (v), and associated com- 

 panion-cells (s). 



Vascular bundles in which the xylem and phloem run alongside 

 one another, as in the stems of Dicotyledons and Monocotyledons, 

 are called collateral. The xylem is usually directed centrally, and 

 the phloem peripherally in the stem. The case of the Cucumber, 

 where extra phloem adjoins the protoxylem, is described as hi- 

 collateral (Fig. 25). When a cambium is present the bundle is 

 described as open (Figs. 23, 25), when cambium is absent it is closed 



(Fig- 31). 



The uniformity of the cylindrical structure of the stems of Flowering 



Plants is very striking. The reason for it is to be found in the fact 

 that it satisfactorily meets the requirements. The stem has at once 

 to serve for the physiological transfer of material, and for the mechani- 

 cal support of the leaves and branches. The cylindrical form, or 

 even the hollow cylinder serves these purposes well. A parallel may 

 be drawn with bones. The marrow-cavity corresponds mechanically 

 to the pith of the stem, while in either case the harder tissue forms 

 an external cylinder. In the case of Birds, however, the bones may 

 be hollow, as in a Grass-haulm. The result in either case is high 

 mechanical strength combined with lightness (see Chapter X.). 



In addition to the tissues thus described, which are generally distributed 

 in the stems of land-living plants, there are others of occasional occurrence. 

 These are often characteristic of certain families of plants. The most im- 

 portant of them are the laticiferous tissues, and various glandular, secretory, 

 and excretory cells or groups of cells, containing crystals, essential oils, and 

 other bodies. The laticiferous tissues consist of continuous tubes widely 

 spread through the parts of the plants that contain them, and filled with a 

 milky white, or sometimes red or yellow juice, called latex, which exudes 

 whenever the tissues are cut, or otherwise damaged. This latex coagulates 

 on exposure to the air, or on addition of certain chemicals. Rubber is 

 prepared from such coagulated latex, the foundation of the coagulum and of 

 Rubber being the complex hydrocarbon, Caoutchouc. Laticiferous tissue 

 exists in two different types. In the first the tubes result from the fusion of 



