210 XVI. THE STEM APEX. 



tracheides which appertain to the leaves are first visible. These 

 join those of the stem-bundle. In Hippuris, therefore, we have 

 a single central vascular bundle, belonging to the stem only, and 

 therefore a " cauline " bundle, with which are articulated the vas- 

 cular bundles appertaining to the leaves the "foliar" bundles. 

 In the axils of the leaves, at a short distance from the apex, 

 small flat protuberances begin to be raised, which are the com- 

 mencement of fan-like scales, each borne upon a short stalk-ceil. 

 Only in specimens in. course of flower-production do we here meet 

 with the commencing formation of axillary buds ; vegetative 

 branching in Hippuris is rare. 



In order to become better acquainted with the structure of the 

 growing apex, we select a good median longitudinal section, and 

 treat it with Eau de Javelle (potassium hypochlorite). Gas 

 bubbles soon begin to escape from the preparation. The action 

 must last a shorter or longer time according to circumstances. 

 The most beautiful results are obtained with sections of alcohol 

 material. The Eau de Javelle dissolves out the plasmic cell- 

 contents, while the cell-walls stand out sharply ; the sequence of 

 cells is then easy to follow. As soon as the necessary degree of 

 transparency is attained, the preparation must be washed with 

 water. If the section has become too transparent, it can be 

 partially restored by treatment with a solution of alum, or with 

 alcohol. If grains of lime, which are separated out, should 

 cling to the preparation, dilute acetic acid should be allowed to 

 run in, in order to remove them. The washed preparation can 

 be preserved in glycerine, but must be first laid in very dilute 

 glycerine, and this concentrated slowly in air. In some cases 

 the preparation is improved by slight staining of the walls with 

 vesuvin, Bismarck brown, or safranin. In other cases, as in 

 this, Eau de Javelle can be used when it is desired to dissolve 

 the cell-contents, and make the cell-walls distinct. Cutinised 

 cell-walls, after some time, are attacked by Eau de Javelle. If 

 the cells are very rich in reserve oily materials, or if the structures 

 contain starch in quantity, the sections must lie for a long time, 

 sometimes for days, in the reagent. If Eau de Javelle is not at 

 our disposal, then treat the section with concentrated potash 

 solution, wash this out, and lay it in concentrated acetic acid. 

 After some time we examine it either in acetic acid or in acetate 

 of potash. It is an advantage not to place the section direct 

 upon the object-slide, but to lay it upon a cover-glass placed 



