VII GROWTH OF SPECIFIC ORGANS 799 



view which might unite many isolated pieces of information, such as the need of 

 blade tissue for nitrogenous compounds, the fact that COj is essential for light to 

 exert its leaf-forming action (Wiedow and Von Guttenberg, 1953), and the stimu- 

 lating effect of light on protein formation in isolated chloroplasts (Stephenson et al., 



1956)- 



The leaves of monocotyledons differ in that they do grow in area in the dark; 

 in fact the length of etiolated oat or barley leaves is abovit as great as that in the 

 light, though the width is definitely less. In one or two cases, notably Narcissus, 

 the leaves are actually longer in the dark than in the light. There is some anatomi- 

 cal evidence that in the early stages of development the leaf primordium of mono- 

 cotyledons is homologous with that part of the primordium of dicotyledons which 

 forms the petiole, i.e. the part which can elongate in darkness. Among the ferns 

 some are said to expand their leaves in darkness, though the pinnae never reach 

 the full size; others, however, behave like dicotyledons. 



The elongation of leaf-veins is evidently controlled by factors different from 

 those acting on the blade or lamina. This was seen early in the development of 

 the auxin work when auxins were applied in lanoline paste to leaves. If applied to 

 veins they would show growth-promotion by curving away from the applied 

 auxin, but the laminar tissue wovild show no response at all. The differentiation 

 is set up while in the late primordium stage, since when auxin is applied to the 

 unopened bud the leaf laminae are afterwards found to be extended and sometimes 

 grown together. A number of interesting abnormalities have been produced in this 

 way (see Thimann et al., 1952, for review). Another way in which the difference 

 in growth control between the veins and lamina is seen is in the results of 

 malnutrition and certain infections. "Frenching" of tobacco leaves, believed to 

 be a virus disease, and "whiptail" of cauliflower, due to molybdenum deficiency, 

 are both diseases in which development of the laminar tissue in the bud is inhibited, 

 while the main vein grows more or less normally ; as a result the leaves are shrunk 

 to a narrow strip, which is not much more than the midrib. The gene "wiry" in 

 tomato has the same effect. 



Such differential response suggests that the more or less linear elongation of the 

 vein tissue is under the control of auxin, while the plate-like enlargement of the 

 lamina, with its indentations (or areas of no growth at all in compound leaves) 

 would be under the control of other growth substances. This conclusion is in 

 agreement with all the observations, though not rigidly proved. In the case of the 

 long linear leaves of grasses and other monocotyledons it is difficult to apply, since 

 their elongation is not light-sensitive and yet apparently not controlled by auxin 

 either. What are the controlling factors for leaf growth? 



In the first place there is no doubt that auxin is produced in leaves, and trans- 

 ported polarly in them from the margins to the central veins and from apex to base 

 therein. The production is dependent on light and the auxin content falls pre- 

 cipitously when the plants are placed in darkness. Correspondingly more auxin 

 is formed, other conditions being equal, in long days than in short. The auxin 

 yield per unit area, and sometimes per leaf, is greatest in the youngest leaves and 

 decreases steadily with increasing age. On the other hand the role of this auxin 

 in leaf growth is uncertain. Correlation needs to be made between the temperature 



Literature f>. 8i6 



