CHAPTER II 



THE EVOLUTION OF PURE OXYGEN UNDER LIGHT 



The aquatic plant Hydrilla verticillata — Anatomical characteristics — 

 Analysis of pond -water — Proportions of different gases absorbed by 

 water — Method of obtaining evolution of unmixed oxygen — Absorption 

 of oxygen by water. 



In experimenting with various water-plants, I found 

 Hydrilla verticillata to be the most suitable for the investiga- 

 tion of photosynthesis. It is a flowering plant, having a 

 length of about 80 cm., bearing a number of leaves in whorls 

 at the nodes of the stem. The average area of the leaf is 

 about 25 sq. mm. ; the internodes are short towards the 

 apex, where the young leaves are crowded together. There 

 are two varieties met with : — the stem in the one is relatively 

 thin, about 1 mm. in diameter, and bears from four to five 

 leaves at each node : the diameter of the stem of the thicker 

 variety is about 3 mm., and there are about nine leaves at 

 each node. A transverse section of the leaf and the stem 

 of the plant is given (fig. 1) ; the lamina consists of only 

 two layers of cells, except at the midrib. The upper layer, 

 which faces the sky, consists of relatively large-sized cells 

 containing chloroplasts, the diameter of each cell being 

 about 0.08 mm. The cells of the lower layer are smaller 

 in size. The chloroplasts in the cells are shown in the left 

 half of the figure ; those in the cells of the right half have 

 been decolorised and stained with iodine. This treatment 

 shows about forty starch-grains in each cell the leaves 

 having been previously exposed to light ; the starch-grains 

 disappear from the cells during prolonged darkness. 



A section of the stem is also given in the figure ; it shows 

 about three layers of chlorophyll-containing cells at the 



