56 PRACTICAL BOTANY. 



constituents are cells arranged in radial rows, with 

 thin cellulose walls (blue Schulze's solution), and plen- 

 tiful 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 

 outwards 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 cf. the Elm and 

 Pine, which, being lignified stems, and having more 

 definite secondary increase, are better types for the 

 study of cambium. 



C. The Xylem also consists of elements of various 

 structure : of these the most noticeable are 



a. The Vessels, easily recognised by their large 

 cavity: they are arranged in radial rows, the indi- 

 viduals decreasing in size towards the central limit 

 of the bundle. The walls are thick and lignified 

 (yellow with Schulze's solution, or with H 2 S0 4 and 

 aniline sulphate), they have no protoplasmic contents ; 

 their further distinctive characters can only be seen in 

 longitudinal sections. Thyloses may be observed, [cf. 



