SPECIFIC GROWTH FACTORS 337 



appear is determined entirely by the water level. The internodes 

 in water are longer than those in the air, thus adapting the plant 

 to the depth of the stream or pond. Houseleek (Sempervivum) in 

 dry air is a rosette plant with succulent leaves. When given plenty 

 of water and grown in a humid atmosphere, internodes appear 

 and the leaves are more nearly normal, but these variations of 

 form with humidity and transpiration have already been discussed 

 (Chap. XX). In the case of apples, it has been noted that the 

 lack of water makes the fruit more flat or compact and less elon- 

 gated, so that even fruits may be changed by the water supply. 



Owing to the close relation between light and transpiration, it 

 may be difficult to separate an effect caused by light from one 

 caused by humidity. The two factors work together to produce a 

 common effect. Thus in Convolvulus arvensis Magocsy-Dietz (1920) 

 found four distinct types of leaves depending upon the combination 

 of the two factors of radiation and moisture: (a) auriculate or 

 lanceolate leaves, in dry and moderately sunny places; (b) hastate, 

 in shady and moderately damp places; (c) sagittate in bright sun- 

 light; and (d) elliptical, in diffuse light and rather damp places. 



Oxygen. — The effect of oxygen upon respiration has been pre- 

 viously described (Chap. XXIV). Here it only remains to add that 

 Jaccard (1893) noted that any change in the oxygen pressure above 

 or below normal stimulated growth, provided the change was not 

 too great nor too prolonged, in which case growth was diminished. 



The form of the plant, especially in the cryptogams, may also 

 be controlled by regulating the oxygen content of the environ- 

 ment. In the common mold (Mucor), the hyphae which form the 

 ordinary mycelium break up into short cells resembling yeast if 

 oxygen becomes deficient, thus forming the "mucor-y easts." 



Food. — The amount of food present is, of course, a factor in 

 determining growth, but it has seemed advisable to consider this 

 subject under the conditions which determine the manufacture 

 of food, — light, water, etc. 



Nutrient Salts and Ions. — The effects of the salts and their ions 

 have already been discussed in the early chapters of this book 

 (Chaps. IX and X); but, in this connection, attention should be 

 called once more to the importance of hydrogen ions in the growth 

 processes of plants (Chap. XI). This frequent resort to cross 

 references should serve to emphasize the interrelation of all the 

 processes met with in the life of plants. Although we may first 



