THE STEUCTUEE OF THE LEAF 



387 



If the stem is polystelic in its arrangement, the leaf may 

 receive one or more complete steles from it. Each has then 

 the same structure as that of the axial steles, possessing a peri- 

 cycle and being snrromidecl by an endodermis continuous with 

 those of the stem. If the stem is monostelic, each leaf contains 

 one or more steles, usually called meristeles, which by subsequent 

 differentiation of the tissue of the periblem of the stem become 

 united with the stele of the latter. The vascular bundles in 

 each meristele may be sarrounded by a general endodermis and 



-ep 



Fig. 772. Mesopodium (petiole") of I'rimula witb a single meristele. ep. 

 Epidermis, co. Cortex, eit. Endodermis. pp. Pericycle. ph. Phloem. 

 ■r. Xj'lem. 



pericycle, or the meristele may be replaced in a sohizostelie 

 manner by a number of strands, each consisting of a single vascu- 

 lar bundle with endodermis and pericycle enclosing it {fig. 771, a). 

 In either case the pericycle very frequently contains a good deal 

 of sclerenchyuia, forming strands behind or in front of tlie 

 bundles. Rarely schizostelic structure is found in the petiole 

 of a polystelic stem, as in Auricula. 



We may find different modes of arrangement of the bundles 

 in the meristele. In the first and most common case they do 

 not form a complete ring, but an arc not closed towards the 



