FIBROVASCULAR TISSUES: PHLOEM 123 



longitudinal section. The soft bast consists of parenchyma, sieve 

 tubes, and their companion cells. Crystallogenous elements are 

 inconspicuous in the bast of the grapevine. The sieve tubes in 

 some cases show their terminal and mainly functional sieve 

 areas in transverse section. The plate in this instance is blocked 

 by a formation of callus which in the vine covers the plate each 

 autumn and disappears again in the following spring when 

 vegetative activity is renewed. This condition is unusual, since . 

 quite generally in the angiosperms, as in the gymnosperms, the 

 appearance of a callus marks the end of the life of the sieve 

 tube. A seasonal callus is likewise found in the barberry and in 

 certain other instances. The final callus, marking the dissolution 

 of the characteristic functional elements of the phloem, is known as 

 the definitive callus. The tubes of the phloem in the grapevine 

 are of interest in showing a considerable amount of so-called transi- 

 tory starch, indicated in the figure by the larger black granules. 



Both the xylem and the phloem of herbaceous dicotyledons and 

 of the monocotyledons show interesting modifications of the con- 

 ditions characteristic of arboreal and perennial forms. Here the 

 vessel and the sieve tube constitute to an increasing extent, and 

 sometimes exclusively, the functional elements of the two important 

 regions of the fibrovascular bundle and are present in what may be 

 regarded with a strong degree of probability as their highest and 

 most specialized form. The vessel is practically always porous in 

 herbs and presents as a usual peculiarity of organization terminal 

 walls which are nearly or quite horizontal. The sieve tube also 

 tends toward horizontal end walls, and the sieve plates have prac- 

 tically disappeared from the lateral walls, the terminal areas alone 

 being functionally important. 



In concluding the statement regarding the organization of the 

 structures in the phloem it will be well to represent side by side 

 the stereoscopic aspects of the several types of sieve tubes in com- 

 parison with corresponding varieties presented by tracheids and 

 vessels. In Fig. 940 is shown diagrammatically the organization of 

 a sieve tube in the secondary phloem of a gymnosperm. It is clear 

 that the plates are confined to two opposite walls, which are the 

 radial ones. The cell, further, tapers imperceptibly to a point at 



