692 . BIOLOGICAL EFFECTS OF RADIATION 



new shoots emerged from the soil as offsets. In similar tests Oden 

 (52), by lengthening the autumn day to 17 hr., caused the usual rapid 

 autumn leaf fall in Acer campestre, Lonicera Periclymenum and Viburnum 

 Opulus to be replaced by continuous dropping of the leaves. After 

 normal leaf fall the added illumination caused the new buds to unfold and 

 new leaves were developed. 



In first-year cultures Mochkov (49) found that Rohinia Pseudo- Acacia 

 L., a representative of low^er latitudes, grew slowly from the outset in a 

 10-hr. day, but in the full day rapid growth continued till stopped by 

 cold, so that the wood did not ripen. On the other hand, Salix lanata 

 L., a representative of high latitudes, grew more rapidly in the 10-hr. 

 day but soon terminated its activities, while in the full day growth con- 

 tinued for a longer period and ultimately w^as much greater in amount. 

 Nevertheless, the long-day cultures became inactive long before winter 

 set in. 



Dexter (16) has recently studied effects of day length on hardening 

 in alfalfa, wheat, cabbage, and tomato at different temperatures. At 

 0°C. wheat and alfalfa hardened more fully in a long day than in a short 

 day and there was no decided indication of elongation. Winter wheat 

 plants previously exposed to a short day at 60°F. hardened more fully in 

 the cold room than plants which had been in a long day when placed in 

 either a long- or a short-day hardening treatment. At higher tem- 

 perature a short light period interfered seriously with the hardening 

 process only in winter wheat, but a long light period caused elongation of 

 foliar parts in each of the species studied. 



MORPHOLOGICAL AND ANATOMICAL EFFECTS 



Following the early work on the subject (23), the fact that the 

 morphology of plants may be variously modified by the day-length 

 factor has been demonstrated by a number of investigators. Doroshcnko 

 (17) found that associated with retarded reproductive activity in wheat 

 and barley, which are long-day plants, when these plants are exposed 

 to a short day, there is a marked increase in leaf development, amounting 

 in some instances to 10 times that under long-day conditions. In flax, 

 another long-day type, increase in basal branching was observed under 

 short-day conditions and there was decreased leaf development. Lubi- 

 menko and Szeglova (42) subjected a number of species to various regu- 

 lated day lengths and on the basis of results obtained the conclusion is 

 reached that for each combination of day and night there exists a special 

 form of the plant from the standpoint of relative development of the 

 different organs. In Hordeum vulgare fruiting occurred only in the 

 full day, maximum production in percentage of total dry weight was 

 obtained in a very short day for the leaf, in the full day for the stem, 

 and in intermediate day lengths for the root. In Benincasa cerifera, 



