SUNFLOWER. 



[CH. IV 



do the work of leaves in assimilating carbon. The cri- 

 terion of the absence or presence of chlorophyll fails 

 therefore, and we are driven to see that the absence or 

 presence of flowers may be of value. 



The obvious external characters of the sunflower stem 

 may be summarised before going on to its microscopic 

 characters. The points to be noted are that it is a 

 vertical structure divided into nodes and internodes, the 

 nodes bearing opposite 1 leaves, or in the case of the 

 lowermost node opposite leafy cotyledons ; it terminates 

 in a growing point and bears buds (which are potential 

 branches) in the axils of its leaves. 



FIG. 22. 



DIAGRAMMATIC SECTION OF THE STEM OF HELIANTHUS. 

 ep, epidermis. /, pericycle-fibres. x, xylem. 



c, cortex. ph, phloem. mr, medullary ray. 



e, endodermis. c6, cambium. p, pith. 



A transverse section of a young Helianthus stem 

 presents certain resemblances to the section of a root, while 



1 In Helianthus tuberosus the leaves are in sets of three, and there is 

 some irregularity in their arrangement in the sunflower. 



