120 THE ANATOMY OF WOODY PLANTS 



are to be seen the summer elements of the wood in contact with 

 the cambium. They are followed by parenchyma and by crystal- 

 logenous cells, empty and heavily pitted on their horizontal walls. 

 Next, the fibers of hard bast meet the view, and these pass in suc- 

 cession into bast parenchyma, characterized by heavy pitting, 

 abundant starch, and protoplasmic contents, and by sieve tubes of 

 large lumen and scanty parietal protoplasm. Within, in the walls 

 of the sieve elements, lie the companion cells, distinguished by their 

 slender form and the absence of starch in their protoplasm. The 

 sieve tubes in the radial aspect present to the eye strongly inclined 

 end walls which may be compared with the similarly slanting 

 partitions in the vessels of the wood. The sieve element is, in 

 fact, the exact analogue of the vessel, and, just as the vascular 

 elements in angiospermous woods play the main part in the trans- 

 port of water, so the sieve tubes perform the principal role in the 

 movement of the elaborated organic stuffs from the leaves. Another 

 feature of analogy between the sieve tube and the vessel is not 

 only the more or less inclined terminal walls at angles to the lateral 

 ones, but also the function of transport specially provided for in 

 connection with the ends of the element. 



In the vessel this condition finds its expression in the develop- 

 ment of scalariform or porous perforations and in the case of the 

 sieve tubes by the appearance of particularly extensive and large- 

 pored sieve plates on the terminal inclined walls; and these in 

 highly specialized forms, such as herbaceous dicotyledons and 

 monocotyledons, may be the only functional regions. Following 

 the sieve tubes is a zone of bast parenchyma, succeeded in turn by 

 another zone of crystallogenous cells and hard-bast fibers. 



In Fig. 92 is shown the tangential longitudinal aspect of the 

 phloem in the basswood. Here the rays naturally appear in trans- 

 verse section and are clearly of diverse sizes, but are all composed 

 of strongly pitted cells containing abundance of protoplasm and 

 starch. In this aspect the plane of section is purposely slightly 

 oblique so as to exhibit all categories of constituents of the phloem 

 at the same time. To the left lie crystallogenous cells near a large 

 ray. The ray is in contact on the opposite side with parenchyma of 

 the soft bast. Then follow sieve tubes and their related com- 



