EFFECTS OF HIGH-INTENSITY IRRADIATION 123 



Conversely, other phenomena, hke the partial defoliation experi- 

 ment, also show correlations with the light factor. Under high light 

 intensities, a lettuce plant builds a head consisting of round leaves. The 

 growing point is covered by leaves and doubtless receives a much lower 

 light intensity than the outer leaves. Nevertheless, it produces "high 

 Hght intensity" leaves. If a plant that has formed a head is placed under 

 low hght intensities, the growing point again starts building long and 

 narrow leaves which protrude from the head. The nature of these 

 correlations is still unknown. 



An elongated leaf is fiat, and the lamina can be cut at either side of 

 the midrib, retaining its original shape. A round ("high light inten- 

 sity") leaf is strongly folded, and, when cut alongside the midrib, the 

 lamina can easily be stretched to about twice the length of the midrib. 

 Thus, it is mainly the growth inhibition of the midrib that causes the 

 rounded leaves. 



Bensink has studied cell shapes and sizes and finds that the cells on 

 the lamina of broad and of narrow leaves do not differ much in shape. 

 In the midrib, however, differences do exist. In a broad, folded leaf, 

 the cells are much shorter at the base of the midrib than at its top. In 

 an elongated, fiat leaf, on the contrary, the cells at the top of the mid- 

 rib do not differ much in size from those at its base. 



De Lint has recently studied morphogenetic changes of the leaves 

 of lettuce plants in the colored light cabinets, in combination with 

 gibberellic acid. The effects are reminiscent of those in Hyoscyamus, 

 inasmuch as high doses of GA produce strong elongation of leaves and 

 internodes in all colors. The reaction obtained with the different colors 

 in the absence of GA has not yet been sufficiently studied. The effect 

 of differences in intensity in the various spectral regions is still to be 

 analyzed in greater detail. An effect that deserves special attention in 

 view of its difference from that in Hyoscyamus is that red with much 

 infrared does not seem to produce more (rather, even less) elongation 

 than pure red, and the effect of GA does not seem very manifest in the 

 red + infrared cabinet (Fig. 8) and the plant with the highest dose 

 probably is not quite representative. It is clear that, in general, GA 

 produces a low light intensity type. This is even more obvious in an 

 experiment in white light, done at equal light intensities and with dif- 



