182 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[ 33. 



verse section, is due a variation in the number, and to some extent 

 also, in the size of its constituent elements, more especially in the 

 wood. 



The variation in structure of a vascular bundle can be well 

 observed in connexion with the foliage-leaf. The number of 

 bundles entering the leaf (petiole, when present) from the stem 



FIG. 139. .4 Transverse section of an open conjoint, collateral, vascular bundle in the 

 stem of the Sunflower. M Pith. X Xylem. C Cambium. P Phloem. R Kortex ; s 

 Small, and ' large spiral vessels (protoxylem) ; t pitted vessels; t' pitted vessels in course 

 of formation from the cambium; 7i wood-fibres; sb sieve tubes; b fibres of tHe hetero- 

 geneous pericycle ; e endodermis or bundle-sheath ; ic inter-fascicular conjunctive tissue. 

 B Radial vertical section through a similar bundle (somewhat simplified) lettered like the 

 former. (x!50). 



varies from one to many ; these bundles, when derived from a 

 monostelic stem, are segments or branches of the stele (meristeles), 



