ULTRA-VIOLET AND SEED PLANTS 877 



Gola (32) found that green leaves exposed to 3400 to 3800 A showed 

 an intense fluorescence. Certam white flowers exhibited a similar 

 phenomenon, which he thought might be due to the remains of traces of 

 green plastids once present, or of their degradation products. He sug- 

 gests that fluorescence is associated with lipoid-like substances. 



Cecco (10) using Petri's method for isolating fluorescent substances 

 from various parts of numerous plants on filter paper found also an 

 abundance of a fluorescing substance in the green organs of plants, and in 

 those parts capable of becoming green. She states that the presence of 

 fluorescing substances in the assimilating tissues seems to constitute a 

 normal condition of vital activity. She points out that there cannot be 

 attributed to this fluorescence a protective function against solar radia- 

 tion of short wave-length since there is no relation between the relative 

 amount of the fluorescing substance present and the ability of the plant 

 part to withstand radiation. The fluorescing substance was thought to 

 be of the nature of a glucoside. 



Vodrazka (120) noted the fluorescence in the cross sections of various 

 woody stems when these were exposed to ultra-violet radiation, and 

 thought that the causes of the fluorescences in Robinia were substances 

 related to tannins. This hypothesis is similar to that of Cecco, in that 

 Cecco found some tannic substances in her fluorescing extractions. The 

 fluorescing substances were found regularly in certain parts of the pith, 

 primary wood, heartwood, and sapwood. 



EFFECTS OF ULTRA-VIOLET RADIATION UPON ANATOMICAL STRUCTURE 



AND FLOWTER FORMATION 



Since the penetration of the shorter ultra-violet rays is probably not 

 very great, one would not expect them to have any considerable direct 

 effect on the anatomical structure of plants except for the destructive 

 action to the superficial cells. There is, of course, the possibility of 

 chemical changes being brought about in the superficial layers which 

 might in turn affect deeper lying cells, but we have no evidence for this. 

 As for the longer, more penetrating, ultra-violet rays there is no evidence 

 of their effectiveness in this field either. 



Pfeiffer (65) reports that plants growTi in daylight under Noviol "0" 

 glass which cuts off practically all ultra-violet are less stocky, less sturdy, 

 more watery, and weaker in vascular development than those grown under 

 Corex glass which transmits all wave-lengths of solar ultra-violet. 



From the fact that stem diameters, stem heights, fresh weights, dry 

 weights, percentage of moisture, and chemical analyses of plants grown 

 under similar conditions by Popp (71) showed no marked differences we 

 should not expect to find marked internal anatomical differences. Such 

 differences were not found by Popp. The difficulty in obtaining sufficient 

 and comparable material for anatomical study under such conditions 



