344 BROOKLYN BOTANIC GARDEN MEMOIRS 



tional advantage is that the leaves are so thin and transparent that 

 they may be placed under the microscope and the details of cell struc- 

 ture studied with care before the cells are injured or treated with 

 reagents. 



In a typical leaf cell the cytoplasm is transparent and nearly color- 

 less, with a few granules, while the nucleus is only slightly less trans- 

 parent and as a rule shows a few granules and a nucleolus. When a 

 leaf is mounted in a drop of water under a cover glass the cells remain 

 unchanged in appearance for hours. 



If an intact portion of the leaf is cut or crushed the cells in the 

 neighborhood of the injury soon change their appearance. In the 

 course of five or ten minutes the nuclei of the cells nearest the injury 

 assume a more granular (or vacuolated) appearance and soon begin to 

 darken. The darkening does not begin at the surface but appears to 

 take place almost simultaneously throughout the whole mass of the 

 nucleus. Not until the nucleus has become very dark (so as to stand 

 out very conspicuously when the preparation is viewed under the low 

 power of the microscope) does the cytoplasm begin to darken per- 

 ceptibly. It may be several hours after the nucleus has darkened 

 before a change of color can be perceived in the cytoplasm. (This 

 is also true where the thickness of the cytoplasm has been increased by 

 plasmolysis so as to be as great as that of the nucleus.) The darkening 

 of the cytoplasm does not seem to be more rapid at the surface than 

 elsewhere. 



That the darkening is due to oxidation is shown by several facts. 

 Among these the following may be mentioned. 



I. A microscope slide is smeared with vaseline, a leaf is laid upon 

 the vaseline and more vaseline is carefully placed upon the leaf. A 

 small splinter of glass (from a broken slide) is placed on the leaf and 

 another slide is gently pressed upon it, so as to spread the vaseline and 

 bring the glass splinter close to the leaf without injuring the latter. 

 Care should be taken that any air bubbles which may be included in 

 the vaseline are not in contact with the leaf in the neighborhood of 

 the splinter of glass. 



The leaf is left over night in order that the oxygen present in the 

 intercellular spaces (or adhering to the surface of the leaf) may be 

 used up by respiration. On the following morning the upper slide is 

 pressed down with sufficient force to drive the splinter into the leaf 

 and crush it. It is then placed on the stage of the microscope and kept 

 under observation. It is found that while some darkening occurs it 

 is at first largely confined to the drops of juice forced out of the leaf 

 by the crushing (the juice seems to spread along the fibro-vascular 

 bundles in some cases). The darkening of the nucleus and cytoplasm 

 is usually much slower than in air (especially with fresh leaves). 



