CH. VI] SECONDARY PHLOEM. 87 



are not always the same. In the case of the xylem, the 

 cambium in the spring develops large vessels, while 

 in the autumn smaller elements are produced It is a 

 corresponding series of changes that gives rise to the 

 alternate layers of tissue in the secondary phloem. These 

 layers in the bark are distinguished by a physical 

 character, namely, hardness, and are described as hard and 

 soft phloem. 



Fig. 39 represents a transverse section of oak bark 

 highly magnified. At the upper end of the drawing 

 (which represents the outer side of the section) is the cork 

 ck, and phellogen phel ; at the lower edge of the drawing 

 (inner side of the section) are seen the medullary rays 

 running in radial lines. The layers of hard phloem 

 run concentrically at right angles to the medullary 

 rays, separated from each other by concentric layers of 

 soft phloem. Outside the region of alternate hard and 

 soft phloem, and inside the periderm, is the original 

 cortex, the limits of which are not clearly distin- 

 guishable in transverse section. In the longitudinal 

 section (fig. 40) the cells of the soft phloem are seen to 

 differ in size and shape from those of the cortex. The 

 cells, which together with sieve-tubes are the essential 

 elements of the soft phloem, are rich in tannin, a fact 

 which is familiar from a practical point of view in the use 

 made of oak bark by tanners. The structure of the hard 

 phloem will be understood from fig. 40; it consists of 

 elongated pointed fibres with thick walls and very minute 

 cavities. The layers of phloem fibres are bordered, as may 

 be seen in longitudinal section, by rows of cells, each 



