40 PRACTICAL BOTANY 



ii. The soft bast, or phloem, consists of elements of very 

 different structure and function : these are 



a. Sieve-tubes, which appear in transverse section as the 

 larger cavities of the soft bast : their walls are rather thin and 

 consist of cellulose (blue, chlor-zinc-iodine). Occasionally 

 these cavities will be found traversed by transverse septa, 

 having a punctate appearance. These stain dark brown with 

 iodine solution : they are transverse sieve-plates. (See the 

 description of sieve-tubes in Cttcurbita.} 



b. Abutting directly on the sieve-tubes, and appearing as 

 though they had been cut off from the sieve-tube by a longi- 

 tudinal wall, may be seen smaller cells : these are the com- 

 panion-cells, but they are not readily distinguished. 



c. The remaining elements resemble the sieve-tubes in 

 transverse section except in their smaller size, and absence 

 of sieve- plates : these are cambiform cells, or phloem- 

 parenchyma. 



Passing inwards, the distinction of these several constituents 

 of the soft bast becomes more difficult, while the walls are 

 thinner, and the arrangement of the elements is more regularly 

 in radial rows, till, in the band of thin-walled tissue which 

 borders immediately on the xylem, these characters become 

 very obvious. The 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 trans- 

 verse 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 

 cambium 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 development of the different tissue-? 



