90 TRACIIEAKY ELEMENTS 



large, oval terminal perforations. As in the vessel elements of 

 Bctula and Liricxlendron, various types and patterns of pits are 

 characteristic of the lateral walls. A study of the short cylindrical 

 type of vessel element, with greatly-enlarged simple perforations 

 in the transverse end-walls, may be made Avitli the aid of stem- 

 sections and macerated xylem of the pumpkin {Cucurbit a) . As 

 seen in macerated tissue, the individual vessel elements are some- 

 what drum-shaped with densely-pitted lateral walls. The 

 enormous diameter of these vessel elements and their union to 

 produce vessels may readily be studied in hand-sections stained 

 with phloroglucinol and hydrochloric acid. For a description 

 and illustrations of the development and structure of vessels in 

 Cucurhita reference should be made to the work of Esau and 

 Hewitt (1940). 



5. The cellular composition of paper. A large proportion of 

 paper is obtained from the wood of certain gymnosperms (e.g., 

 Abies, Picea) and angiosperms (e.g., Populus, Betula). The 

 first stage in the manufacture of pai)er from wood consists in 

 the mechanical and cliemical maceration of the x.ylem which re- 

 sults in the partial dissociation of its component cells. This 

 macerated xylem is commercially known as tvood pulp and after 

 being bleached, colored or "sized," according to the required use, 

 is compressed under great pressure into paper sheets (cf. Kellog, 

 1923 for further details). It is interesting to note, however, 

 that despite the drastic treatments involved in the production of 

 wood pulp, many of the tracheary elements are well preserved 

 and their structure and pitting is recognizable if small pieces of 

 soaked paper are carefully teased apart in water and examined 

 under the microscope. ]\Iake a study of the various types of cells 

 found in newspaper, blotting-paper and some cheap grade of 

 writing-paper. 



IV. Suggested Drawings and Notes. — 



1. Prepare drawings to show llie shape, structure, and ar- 

 rangement of the ])riniary xylem tracheae in the vascular bundle 

 of the bean hypocotyl or the stem of Trifoliuw. as seen in trans- 

 and longi-sectional view. These diawings shouUl be supple- 

 mented by showing, on a l;irge scab', the details of small jjortions 

 of the secondary wall-patterns of isolated elements of the proto- 



