Criteria of Successful Processing 713 



cndally, producing harsh stainino; effects. Note especially the collapsed 

 plasma membranes in the epidermis and chlorenchyma. This em- 

 bedded material was brittle and only the pieces from the upper three 

 internodes could be cut without serious tearing. 



Figure 11.2r is from a third batch of alfalfa stem which was proc- 

 essed exactly like the material from which Fig. 11.2a was obtained. 

 The structure of the deeper cortical cells is well preserved, but the 

 epidermis shows extensive peeling away from the chlorenchyma. The 

 condition of the protoplasts indicates that the killing fluid and sub- 

 sequent processing were not at fault. It is probable that the peeling 

 of the epidermis was caused by compression of the extremely soft 

 hypodermal tissues when the fresh stem was cut into pieces for killing. 



Roots are somewhat more difficult to judge than organs containing 

 chloroplasts. Root cells that contain leucoplasts should be examined 

 for the position of these plastids. If the plastids are inconspicuous, the 

 condition of the thin plasma membrane will indicate whether plas- 

 molysis has occurred. 



The critic must be much more lenient in the examination of sec- 

 tions of wood. Boiling the wood in water, desilicification with hydro- 

 fluoric acid, and infiltration under pressure for long periods are not 

 conducive to preservation of protoplasmic or even cell-wall details. 

 Mechanical tearing can be easily recognized. In the nonliving elements 

 (tracheae and tracheids) the concentric layers of the thick wall 

 should not be separated. The innermost layer is often found to be 

 completely separated in poor preparations. Parenchymatous elements, 

 such as wood rays, diffuse wood parenchyma, and epithelial cells of 

 resin canals, are subject to plasmolysis. Perfect fixation of these paren- 

 chymatous cells shotdd not be expected. 



The embryo sac of lily is a difficult subject that tests the effective- 

 ness of a method and the skill of the worker. The young megasporo- 

 cyte of lily is comparatively easy to preserve in good condition. Such 

 fluids as chrome-acetic and Bouin's, followed by the traditional alco- 

 hol-xylene series, yield excellent preparations and a good ratio of 

 well-preserved sporocytes. The sporocyte and integument initials in 

 Fig. 11.3 are preserved very well indeed for routine class material. 



With continued enlargement of the sporocyte the cytoplasm prob- 

 ably becomes highly fluid, the integuments elongate as thin sheets of 

 tissue, and these structures become increasingly subject to damage. 

 Look for evidence of plasmolysis of the sporocyte and wrinkling of 

 the rims of the integuments. In Fig. 11.4rt the finely granular cyto- 

 plasm is obviously not shrunken, and the rounded rims of the integu- 



