ELM. STEM. 73 



are, however, difficult to distinguish in transverse 

 sections. They are : 



a. Sieve - tubes, which are nearly circular in 

 section, and usually of larger cavity than the other 

 constituents. 



1). Bast-parenchyma : cells often arranged in more 

 or less regular radial rows : certain of the cells differ 

 from the rest in containing one or more crystals. 



The nature of these several tissues will be more 

 successfully studied in longitudinal sections. 



6. The Cambium consists of thin-walled cells ar- 

 ranged, as in the Sunflower, in radial rows, which 

 may often be traced outwards into the phloem, and in- 

 wards into the xylem : the cells have copious protoplasm, 

 in which a nucleus may often be observed. 



Note that the tangential walls are thinner than the radial 

 walls ; also that the radial diameter of the cells is less than the 

 tangential. These facts, together with the arrangement of the cells 

 in radial rows, point to a sequence of divisions, by walls parallel to 

 one another, in a tangential direction. If careful comparisons of 

 a number of different radial series be made, it will be found that 

 the arrangement is such as would result from the action of Sanio's 

 law of cambial division (compare Pinus, p. 141). 



7. The xylem also consists of several different tissue- 

 forms, all of which have lignified walls (cf. reactions). 

 They are: 



a. Vessels, easily recognised by their large cavity, 

 and by the absence of any protoplasmic body. They 

 occur, singly or in groups, scattered through the xylem. 



It may be found that the cavity of some of the vessels is filled 

 with a cellular tissue. This is especially frequent in the part of 

 the xylem-ring nearer to the centre. The name thylose is given 

 to such cells (see above, p. Cl). 



