PRACTICAL WORK 287 



{5) Sieve tubes and companion cells. Examine the demonstra- 

 tion preparations of sieve tubes and companion cells in trans- 

 verse and longitudinal sectipn of the stem of Cucurbita. Note 

 the thick perforated sieve plates (cross walls of sieve tubes), 

 the callose, the cytoplasm contracted from the side walls of the 

 tube but not from the plates, and the narrow companion cells 

 (protein cells) with granular contents and conspicuous nuclei. 

 Measure with the micrometer eyepiece the diameters of a sieve 

 tube, of a companion cell and of the pores of the sieve plate. 



(6) Vessels, fibres and wood-parenchyma. Examine a piece 

 of Cucurbita stem and identify the longitudinally running 

 strands (bundles) with large openings [vessels] visible to the naked 

 eye. Tease a little of a previously macerated bundle from the 

 stem of Cucurbita in a drop of dilute glycerine, and observe the 

 narrow spiral and annular vessels and pieces of the broad reticu- 

 late vessels. Measure their diameters. Thin-walled parenchyma 

 cells are associated with the vessels. Treat similarly a little 

 macerated willow wood and note that it consists mainly of 

 fibres — long narrow cells with thick walls and tapering ends — 

 and of a few vessels bearing close-set bordered pits where they 

 abut on one another. Measure the diameter of a vessel. Note 

 also the thick-walled oblong cells with simple pits running through 

 the wood in plates (medullary rays) . 



(7) Water tissue. Note the massive thin-walled tissue forming 

 the bulk of the succulent leaf of Kleinia (or other " leaf succulent ") . 

 Contrast this with the layer of tissue containing chloroplasts near 

 the surface. The green cells draw on the water tissue when 

 they lose water more rapidly than it can be supplied through 

 the bundles from the roots. 



Examine transverse sections of the fresh Begonia leaf and note 

 the many layers of thin-walled colourless cells on the upper side. 

 This is derived from the upper epidermis by divisions parallel 

 with the surface. These cells are living, and the very thin cyto- 

 plasmic lining of the cell walls can be seen here and there. The 

 water tissue on the lower side contains chloroplasts, and is not 

 derived from the epidermis but from the mesophyll tissue. It 

 is, however, very distinct from the main photosynthetic layer 

 in the centre, which is densely packed with chloroplasts. 



