SUGGESTED DRAWINGS AND NOTES 99 



wall of the cylindrical sieve-tube elements. In critically stained 

 sieve-plates, each "pore" is occupied by a dark central spot, 

 representing- the large plasmodesma and is surrounded by a dis- 

 tinct callus-cylinder stained a light blue. If definitive-callm has 

 not yet appeared, the portions of the plate between the callus- 

 cylinders should appear unstained. Note that in addition to the 

 sieve-tubes, smaller companion cells appear and are distinguished 

 by their nucleated protoplasts and their triangular or quad- 

 rangular form as seen in trans-section. Phloem parenchyma is 

 also present but is difficult to distinguish from sieve-tube ele- 

 ments unless the latter exhibit sieve-plates. This study of phloem 

 should be continued with the aid of longi-sections which likewise 

 should be stained in aniline blue. Note carefully the appearance 

 and structure of the sieve-plates as seen in sectional view, and the 

 relation of the sieve-tube elements to the companion cells and the 

 phloem parenchyma. Often slime-plugs will appear in certain 

 of the sieve-tube elements. They can readily be induced by treat- 

 ing the section with 709^ alcohol. 



2. The phloem of Rohinia and Finns. Mount thin radial and 

 tangential sections cut from living twigs of these genera in a .1% 

 aqueous solution of aniline blue and study under high magnifica- 

 tion the structure and distribution of the sieve-plates and the 

 form and relationship of the sieve-tube elements and sieve-cells. 



III. Suggested Drawings and Notes. — 



1. Diagram the general structure of a bicollateral bundle from 

 the stem of Cucurbit a as seen in trans-section. Label all essential 

 parts and indicate by circles the position of the largest vessels of 

 the xylem. 



2. Draw small portions of the phloem tissue of Cucurhita as 

 seen in both trans- and longi-sectional views. Show carefully the 

 structure of at least one sieve-plate in each drawing. Label all 

 cell tj'pes and structures. 



3. Prepare drawings of small portions of the phloem of 

 Eolinia and Pinus to show the form and structure of the sieve- 

 tube elements and sieve-cells. 



4. Summarize the evidence which indicates that plant viruses 

 may move and be transmitted through phloem tissue (cf. Crafts, 

 1939b, and Esau, 1941). 



