42 Practical Plant Biology. 



If the same experiment had been set up with water which had 

 been previously boiled to remove the carbon dioxide, no bubbles 

 would have been generated at the surface of the plant even when 

 exposed to light. 



The production of oxygen and the conditions governing it may 

 also be studied with this plant. When a small piece is submerged 

 in a glass vessel of water and exposed to light, a stream of tiny 

 bubbles is seen to rise immediately from the cut end of the stem. 

 The stream becomes feebler as we reduce the light, till it stops 

 altogether when the light is very faint. Again the flow of bubbles 

 becomes faster and faster as we increase the light from a faint 

 illumination to bright sunlight. As in the previous experiment 

 no bubbles are formed if the piece of plant is immersed in water 

 deprived of carbon dioxide by boiling. On the screen you see 

 the bubbles rising from the cut end of the stem. The light of the 

 arc lamp which serves to project the image of the plant at the same 

 time furnishes the light requisite for the production of the gas. 



If we expose a culture of yeast, or an organ not having a green 

 colour such as a root or stem, to light under the same conditions 

 as those described above, no evolution of oxygen takes place. 



It is now of interest to examine the tissue of Elodea and see 

 how the green substance is distributed in it. 



The leaves of Elodea are comparatively thin and transparent, 

 being composed of a single layer of cells at the margin, and a 

 small number of layers in the central regions. Down the middle 

 of the leaf runs the mid-rib which is composed for the most part 

 of elongated cells with long passages between them containing gas. 

 When the leaf is submerged and viewed by transmitted light these 

 passages look dark owing to the reflection of the light from their 

 surfaces. These passages give off branches between the other 

 cells of the leaf. The cells bordering the passages are roughly 

 rectangular in form, as you see shown in this diagram. They 

 possess a transparent cell-wall so that the contents within may be 

 plainly seen. The cell-wall is lined by a film of colourless pro- 

 toplasm of variable thickness which in turn surrounds a large 

 colourless vacuole. Embedded in the layer of protoplasm are 

 numbers of tiny ovoid green masses. These are the chloroplasts 

 which confer the green colour on the leaf, and like the chloroplast 

 of Chlamydomonas, are responsible for the formation of starch 

 and for the production of oxygen. Embedded somewhere in the 

 protoplasm a larger colourless body, the nucleus, may also usually 

 be seen. It appears to possess a very definite limiting membrane 

 within which may be seen a bright speck, the nucleolus. Con- 



