212 THE PHYSIOLOGY OF PLANTS 



of rest. A saturation of the air with water vapour, especially 

 if the temperature is low, is favourable to the activity of 

 Fungi of decay in their attack upon plants. The activity 

 of the latter is decreased by the moisture of the atmosphere, 

 as assimilation and transpiration do not then take place to 

 any great extent. For the fungal spores, however, this tem- 

 perature is sufficient, and their development is accelerated by 

 the great moisture. 



If the temperature of green-houses is raised in the winter 

 when illumination and aeration is insufficient, then the plants, 

 stimulated by the moisture to further growth, may develop 

 along wrong lines. The different processes which take place 

 in a leaf make different demands upon the different factors, 

 such as light and heat. Thus the elongation of cells can take 

 place in feebler light than can the assimilation performed by 

 the leaves. The formation of sugar can take place at tem- 

 peratures at which it is no longer used up in the process 

 of respiration ; sugar may therefore often be stored up at low 

 temperatures in leaves, tubers, and other organs. 



It often happens also that in green- houses the temperature 

 is high enough to permit of an elongation of the cells, but 

 the light may not be sufficiently intense to cause any appre- 

 ciable assimilation. If under these conditions the pots are 

 copiously watered, while only a small amount of evaporation 

 takes place from the leaf surface, a considerable accumulation 

 of water will occur within the plant, and this will cause a 

 pathological (abnormal) elongation of the cells. Some groups 

 of cells within the tissues of the leaf will begin to elongate, 

 and even grow out into long tubes, but as they receive no 

 material from without for the elongation of their cells, they 

 use up their own cell-contents. Thus the chlorophyll cor- 

 puscles become disorganised, and nothing but small yellow 

 granules will be found. This causes such impoverished por- 

 tions of the leaf to appear more or less yellowish, even by 

 reflected light, or at all events when the leaf is held up to the 

 light. 



Such appearances are of much more common occurrence 

 than was formerly thought, and it must now be regarded as a 

 sign of a superfluity of water. 



