F. W. WENT 291 



light is necessary for the formation of various growth factors and the 

 production of carbohydrates, but in the second place the light also 

 determines the reaction of the tissues to the available growth factors 

 either directly or through activation or destruction of growth factors. 

 Let us first consider peas. When grown in near darkness, pea seedlings 

 develop small etiolated leaves, but when the cotyledons are cut off 

 even this small amount of leaf growth stops. It Ukewise can be almost 

 completely inhibited by complete darkness. This can be interpreted as 

 meaning that phyllocaline is stored in the cotyledons and is activated 

 in the leaves by light. 



In squash a similar behavior can be observed. In darkness seedlings 

 do not produce leaf blades, but with sufficient illumination leaves grow 

 out. Squashes differ from peas in that they require over 100 times as 

 much light to have their phyllocaline activated. 



In mature plants the phyllocaline is no longer stored in the cotyledons 

 but comes from the mature leaves where it is formed under the influence 

 of light (9). Actually in recent experiments (14) it could be shown that 

 light is required both on the mature leaves and on the leaf primordia for 

 leaf growth in squash plants; illumination of either one alone is insuffi- 

 cient to produce leaf growth. 



These facts can be generalized. Seedlings generally do not expand their 

 leaves when grown in darkness. Many seedlings germinating in a dark 

 forest produce only rudimentary leaves, but their stems are decidedly 

 elongated. Thus climbing plants reach light before they have exhausted 

 their cotyledons in useless leaf growth when they germinate in places 

 where there is not enough light to attain the compensation point. 



Helianthits plants produce leaves of different shapes when grown in 

 different degrees of shading. In full light the plants are vigorous and 

 have large heart shaped leaves. In light shade the plants reach approxi- 

 mately the same size, the leaves being only slightly smaller. But with 

 further decreasing light intensity the leaf size decreases much more than 

 the length of the stem. Not only is this true, but the leaf form of the 

 shade plants also is different from that of the sun plants. The leaves of 

 the former are elliptical, and the lateral veins make much sharper angles 

 with the midrib than in the sun leaves. This indicates that mesophyll 

 growth is much more reduced than vein growth in the shade. In the 

 heaviest shade the first leaves are still approximately normal in size; 

 apparently the storage of leaf growth factors in the seed is sufficient to 



