292 PLANT GROWTH SUBSTANCES 



make the first leaves develop, but the successive leaves become smaller 

 and smaller. 



Both the synthesis of the phyllocaline precursor and its transformation 

 into the active material require light. On the other hand the caulocaline 

 complex is relatively independent of light. Auxin production can occur 

 and continue for considerable periods of time in complete darkness, 

 and another factor of the caulocaline complex is formed in the root 

 system in darkness. Therefore we should expect the following effects: 

 shade does not decrease stem elongation but prevents leaf development, 

 and strong light causes expansion of leaves. 



Recent work (10) has shown the effect of photoperiod on leaf shape. 

 This is more fully described by Ashby (2). 



Nutrition. — Much work has been done on the effect of various nutri- 

 tional deficiencies on the growth of plants. In many plants it is easy 

 to distinguish between the various inorganic deficiencies since the lack 

 of each element has certain specific effects on the plants. These effects 

 are partly expressed as mottling or other color characteristics of leaves. 

 In certain cases, however, pronounced differences in development are 

 produced also. The latter interest us most in connection with our 

 subject. One of the most typical results of zinc deficiency is the so-called 

 Httle leaf phenomenon (6). In tomatoes, citrus, and a number of other 

 fruit trees the leaves which develop when the plants are grown with 

 insufficient amounts of zinc have characteristic small and narrow leaves 

 which ultimately will affect the whole growth. This effect on leaf 

 growth may also be produced to some extent by nitrogen deficiency. 

 In Helianthus and tomato plants narrow leaves or leaflets are formed when 

 an insufficient amount of nitrogen is present in the culture solution. The 

 effect of boron deficiency is different and is confined especially to the 

 growing part, which ultimately dies. In Helianthus again potassium 

 deficiency greatly reduces growth in length, although the leaves are 

 still wide and heart-shaped. Similar differences in the leaves of Pota- 

 mogeton were described by Pearsall and Han by (20). Thus it becomes 

 evident that the nutrition experiments also give clear indications of 

 differential leaf, vein, and stem growth. 



Viruses.— h general analysis of the effects of viruses on plants shows 

 that they can be divided roughly into three groups of symptoms: 



I. Mottling of leaves and stems, due to chlorophyll defects, in severe 



