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. 
1. 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). 
