Murneek 



— 45 — 



Research in Photoperiodism 



sexual reproduction, from flower bud inception to seed maturity, to the 

 same photoperiod or a combination of photoperiod with other environmental 

 factors. In many plants flowers are initiated at one time of the year (one 

 photoperiod) and their further development to anthesis takes place at an- 

 other. It is highly probable that reproduction may commence by exposing 

 experimental plants continuously to specific lengths of day but further de- 

 velopment of floral organs may be curtailed or inhibited under this particu- 

 lar photoperiod. (Borthwick and Parker, 1939; Murneek, 1939). 

 Disregarding equatorial regions, in nature most plants probably have ad- 

 justed themselves not to uniform diurnal periods of light but to continu- 

 ously changing ones. It has been shown that many so-called short-day 

 plants are really short-day -* long-day plants as regards their response to 

 photoperiods. Similarly quite a number of long-day plants are, in fact, 

 short-day —> long-day types. Hence, paradoxical as it may appear, there 

 does not seem to be an essential difference between the two groups (Whyte 

 and Oljhovikov, 1939). 



Cajlachjan (1933) is of the opinion that classification of cereals into 

 spring and winter types is unjustifiable in view of the fact that in any 

 large collection strains exist within varieties, secured from different lati- 

 tudes, that may be arranged in series, from spring to winter forms. Using 

 certain species of Poa, Digitalis, TrijoUnm, Pyrethrum and Hyoscyamus as 

 examples, Krier (1941) claims that there is no clear-cut distinction be- 

 tween winter and spring annuals, biennials and perennials. One type can 

 be converted into the other under environmental conditions as determined 

 by geographic location. 



By considering two stages of reproduction only, flowering and fruiting, 

 Eguchi (1937) recognizes the following classification, some representa- 

 tive plants for each being given by Loehwing (1939). 



Optimal periods for : 



Representative species: — 



Strawberry, Cineraria 



Oxeye daisy, Spring barley 



Physostegia virginiana, Boltonia latisquama 



Soybeans, Cosmos bipinnatus 



Phlox paniculata 



Late rice varieties 



Chrysanthemum osticum 



Spinach, wheat 



Pepper, early rice, buckwheat 



Whether many plants, in flowering and fruiting are as closely adjusted 

 to the light period as would seem to be indicated here, may be questioned, 

 for often changes in length of day merely delay flower development. Then 

 there is considerable evidence extant also that the age of the plant deter- 

 mines in a large measure its sensitivity to length of day (Cajlachjan, 

 1936 ; Purvis and Gregory, 1937 ; Borthwick and Parker, 1938 ; Mosh- 

 Kov, 1939; MiROLjUBOv, 1940). All this seems to suggest the necessity of 

 revision of our conception of photoperiodism. 



The period from flower initiation to their full development (anthesis) 

 should receive at least an equal if not greater consideration than the time of 

 floral inception. Flower differentiation does not always lead to their 



