8 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 95 



In order to obtain further information concerning- the distribution 

 of growth substance in the lighted and shaded portions of unilaterally 

 illuminated coleoptiles, two series of excised Avena tips 3 mm in 

 length were used. The tips were split at the bottom and placed erect 

 upon a thin glass microscope cover slip supported in a paraffin block 

 (fig. i). Small 3 percent agar blocks were attached with 5 percent 

 gelatin to each bottom half of each tip so that one vascular bundle 

 was in contact with each block. One experiment included 12 tips 

 irradiated with an intensity of 5700 ergs/cmVsec. In another, 13 tips 

 were irradiated with 278 ergs from one side, the exposure time being 



Table 4. — Influence of Mercury Arc Radiation on Growth Substance 

 Intact tips illuminated laterally {Avena) 



120 minutes. During this 120-minute period, growth substance moved 

 into the agar blocks in contact with the basal cut ends. These blocks, 

 which acquired growth substance from the illuminated and shaded 

 half tips, were applied then to test plants, alternating " near " and 

 " far " blocks along the rack of Avena stumps. In both series, more 

 growth substance was recovered from the side farther away from the 

 light (table 5). 



Zea mays. — In a manner similar to that employed with Avena tips 

 experiments were performed with excised tips of Zea. These data are 

 also given in table 5. Here again less growth substance was recovered 

 from the side near the light, and, indeed, less from the far side than 

 from coleoptile half tips treated in exactly the same way but main- 

 tained in darkness for the 120-minute period of diffusion. 



