FIBRO VASCULAR TISSUES: PHLOEM 



ill 



tubes, are concerned. Fig. 87 shows a highly magnified view of the 

 region immediately outside and inside the zone of growth, or cam- 

 bium. Internal to the active layer the wood is recognizable by its 

 bordered pits. The initial region, known as the cambium, is dis- 

 tinguishable by the richly protoplasmic character of its cells and 

 their thin tangential walls, which are in contrast with the thick- 

 ness of the radial 

 boundaries. Ex- 

 ternal to the cam- 

 bium lies the 

 phloem, composed 

 most characteristi- 

 cally of somewhat 

 rectangular cells, 

 the sieve tubes. In 

 these, unlike the 

 cambial elements, 

 there is no nucleus, 

 and the scanty 

 parietal proto- 

 plasm presents 

 likewise a striking 

 difference from the 

 richly protoplas- 

 mic cambial region. 



Scattered thinly through the phloem are a few rounded cells 

 which contain protoplasm and grains of starch (represented 

 black as if stained with iodine). The rays cross the longitudinal 

 elements of the phloem, the sieve tubes, and the parenchyma cells 

 at right angles and appear in the figure as single files running nearly 

 straight for a short distance and then pursuing a meandering course 

 outward. The cells of the two rays shown in the figure present 

 a different appearance. In the file of radial parenchyma on the 

 right the elements are filled with protoplasm and show the presence 

 of a nucleus. In addition, a considerable amount of starch is 

 present in the form of grains tinctured black by iodine. It is clear 

 that the cellular elements of the ray to the right continue elements 



FIG. 86. Inner bark with cambium and adjacent 

 wood of the pine. 



