46 BOTANY OF THE LIVING PLANT 



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 vessel? (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, v), and associated 

 companion-cells. 



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 bi- 

 collateral. When a cambium is present the bundle is described as 

 open, when cambium is absent it is closed. 



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.). 



WOODY STEMS OF DICOTYLEDONS. 



If the apical bud be capable of unlimited growth and production 

 of new leaves, and if in the axil of each there is a bud which may grow 

 into a branch similarly endowed, some further provision must be 

 made for the mechanical and physiological support of the enlarging 

 plant. Particularly will this be necessary in plants which continue 

 growing for a number of years, and thus attain large size. In them 

 the requirement is met by secondary growth, through the activity of 

 the tissue called the Cambium. It is present even, in herbaceous 

 Dicotyledons, but it is specially active in enlarging the trunks and 

 branches of shrubs and trees. ' From the bulkv column thus formed 



