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 



C si Jc I J J 



K > /' t k f C 



e ft 



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

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

 small, and ' lai-ge spiral vessels (protoxylem) ; t pitted vessels; t' pitted vessels in course 

 of formation from the cambium; h wood-fibres; 6 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 150) . 



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

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



