SUNFLOWER STEM 69 



the elements is more regularly in radial rows, till, in 

 the band of thin-walled tissue which borders imme- 

 diately on the xylem, these characters become very 

 obvious. This band is 



iii. The cambium, or active formative layer. Its 

 constituents are cells arranged in radial rows, with thin 

 cellulose walls (blue, chlor-zinc-iodine), and plentiful 

 protoplasmic contents : the tangential walls are the 

 thinnest, hence we may conclude that the most recent 

 divisions have been in this direction, and have been 

 repeated. Occasionally traces of recent radial division 

 will be found, but this is less common. The form of 

 the individual cells varies from oblong to square, as 

 seen in transverse section : in the former case the 

 longer axis is tangential. Trace the radial series out- 

 wards into the phloem, and inwards into the xylem : 

 they may often be followed for a considerable distance 

 with certainty. Note how, in passing from the cam- 

 bium to the phloem or xylem, the cells divide, and 

 how the form of the individual cells is modified. 

 Hence we may draw conclusions as to the develop- 

 ment of the different tissue-elements of the mature 

 xylem and phloem from the originally uniform cells 

 of the cambium. For further details see the Elm 

 (pp. 95, 107) which, being a woody stem, and having 

 more definite secondary increase, is a better type for 

 the study of cambium. (Compare Fig. 9, A, p. 107.) 



iv. The xylem also consists of elements of various 

 structure : of these the most noticeable are 



a. The vessels, easily recognized by their large 

 cavity : they are arranged in radial rows, the individual 

 vessels usually decreasing in size towards the central 



