PHOTOSYNTHESIS AND RESPIRATION. 95 



128. Etiolation. The effects of darkness on a green plant 

 in producing starvation and loss of dry weight have already 

 been studied (Art. 65) . Apart from these effects and the 

 non-production of chlorophyll, various changes of form and 

 structure are produced by darkness. 



The stem-in ternodes and the leaf -stalks (if the plant has 

 stalked leaves) are unusually long, giving the plant a 

 " drawn " appearance. In this way there is a chance of 

 shoots reaching the light, as, e.g., in seedlings smothered by 

 other plants. In etiolated plants, also, the leaves remain 

 small and scaly, there is a great development of soft tissue 

 and a meagre formation of hard woody tissue. Large leaves 

 would be useless in darkness ; we might say, therefore, that 

 the plant devotes all its energy to the formation of long 

 " internodes " which may be of use to it. Many examples of 

 etiolated plants will readily occur to the mind of the student 

 e.g. celery (leaf -stalks), grass covered by a roller or a 

 board. 



A green plant must of course have a reserve store of food 

 to draw upon in order to grow in darkness at all. 



* Grow in darkness (in each case with plants of the same kind grown 

 in light for comparison) various plants which have a good store of food 

 in their seeds or in such storage-organs as bulbs, corms, tubers. Sow 

 seeds of Broad Bean, etc. ; plant Potato-tubers, corms of Crocus, bulbs 

 of Onion, Tulip, Narcissus, etc. Also compare the growth in darkness 

 and in light of plants which normally have a tufted or rosette habit 

 (e.g. Crassula, London Pride, "Daisy) and note that in darkness the 

 internodes in most cases become elongated. 



129. The Main Function of the Leaf is to act as a 



laboratory for the manufacture of organic food from the 

 carbon dioxide of the air and from the water absorbed by 

 the roots. To carry out this process energy is necessary, 

 and this energy is supplied by the light which the green 

 chloroplastids absorb. During the process more complex 

 substances such as sugar are formed, and a corresponding 

 amount of energy is stored up in this manner. At the 

 same time a volume of oxygen gas is liberated, equivalent 

 to the amount of carbon dioxide assimilated. Hence green 

 plants exposed to sunlight tend to purify the air rendered 

 foul by the breathing of animals. 



