LATEX-CELLS. 7 



are likely to possess. Much can, however, be done in the ex- 

 amination of the callus by the employment of the reagents 

 mentioned in 282 and 283. The student should not fail to sub- 

 mit a thin section showing the larger cribrose-cells to the action 

 of concentrated sulphuric acid, and remove in this way the whole 

 of the cell-wall, leaving (if the manipulation has been careful) 

 the contents slightly connected together and showing the inter- 

 communication between the cells. 



VII. LATEX-CELLS. 



Latex-cells are abundant for demonstration in many wild and 

 cultivated plants ; but few afford material better adapted to the 

 use of beginners than the greenhouse plant, Euphorbia splen- 

 dens. Other cultivated species of the same genus are about as 

 good. With the younger and softer steins of this plant one has 

 merely to secure thin sections through their outer or cortical 

 portion, when, in a good section, the latex-tubes can be found 

 ramifying irregularl}'. The peculiar dumb-bell shaped grains in 

 the tubes form a characteristic feature. 



When a thin section of any tissue containing latex-tubes is 

 gently heated in a dilute solution of potassic hydrate, or for a 

 shorter time in a stronger solution, the parts become so much 

 softened that the tubes can be easily separated from the sur- 

 rounding tissue, after which they can be floated on to a fresh 

 slide and examined by themselves. 



Abundant material for the study of latex-cells is furnished 

 by plants of the following groups : Lobeliaceae, Campanulaceae, 

 Liguliflorse, and many Papaveracese. 



VIII. SPECIAL RECEPTACLES FOR SECRETIONS. 



These are constantly met with in sections of many stems, 

 leaves, and fruits. A few examples for study are here given. 



(a) Crystal-cells. Look for these in the leaves of the Aracese, 

 Onagraceae, and Chenopodiaceae, and in the bark of almost any 

 of the ligneous Rosaceae (Pomes), where they are especially 

 associated with the bast-fibres. 



(b) Resin-cells and resin-reservoirs are found in the bark of 

 many Conifers- and Umbelliferse, etc., in the leaves of Rutaceae, 

 Hypericaceae, and Myrtaceae. 



(c) Tannin receptacles are found in very many kinds of bark. 

 For the detection of tannin, solutions of potassic chromate or 

 ammonic chromate may be employed, a brown color being 



