86 Practical Plant Biology. 



however, produced and may be evolved if the plant is not exposed 

 to light This gas escapes in solution from the surface of the 

 protoplasm and diffuses away in the water. The production of 

 carbon dioxide is the result of respiration, which in the case 

 of Vaucheria requires dissolved free oxygen. In other words, 

 Vaucheria's respiration is aerobic. It has been pointed out that 

 when carbohydrates are oxidised in respiration the volume of oxygen 

 employed is equal to the volume of carbon dioxide produced. 

 This appears from the equation: C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O 

 and is usually expressed by saying the respiratory ratio is equal 

 to one (CO 2 : O 2 = i). It is evident, however, that when a plant 

 respires oil like Vaucheria, it oxidises a substance poorer in oxygen 

 than those which respire carbohydrates, consequently the respir- 

 atory ratio in the former is less than unity. Therefore oil is a 

 more concentrated form of store material as a supply of energy 

 than starch or sugars. 



When conditions continue favourable for photosynthesis and 

 assimilation of nitrogen, the synthetic or anabolic processes pre- 

 dominate over the wasting or catabolic processes, and a surplus 

 of organic substance is accumulated. This is available for growth 

 and also for the repair of injury. 



If a filament of Vaucheria is wounded the living protoplasm 

 retracts itself a short distance into the tubular wall of the ccenocyte 

 from the point of injury. At the same time the retreating end 

 secretes a covering of cellulose and makes this new portion 

 continuous with the old cell- wall, thus isolating itself from the 

 injured spot. The transverse walls thus formed in response to 

 injury are the only transverse septa found in the ccenocytes of 

 Vaucheria, except those formed in connection with the reproduc- 

 tive organs which will be mentioned later. 



The growth of Vaucheria takes place at the tips of the branches. 

 As was before noticed the ends of the branches are filled with 

 plug-like masses of protoplasm closely packed with chloroplasts. 

 Stained specimens show that the minute nuclei here are subdivid- 

 ing rapidly. The wall covering the rounded end of the branch is 

 somewhat thinner than elsewhere and being gelatinous is more 

 ductile. Consequently it slowly yields here to the osmotic pressure 

 of the dissolved substances in the vacuole pressing the proto- 

 plasmic plug against it. As it yields, the growth of the cytoplasm, 

 the subdivision of the nuclei and chloroplasts and the secretion 

 of more cellulose, where the wall pulls out, keep pace with the 

 elongation. So the length of the branch is increased, while the 

 materials assimilated by the ccenocyte are being constantly used up. 



