PHOTOPERIODISM 501 



flowering response discussed in this and the preceding paragraph appears 

 to be an expression of the capacity of the plant to continue formation or 

 development of floral primordiaupon return to noninductive photoperiods. 



Many plants that are photoperiodically responsive when young later 

 appear to become more indeterminate. Examples are the long-dark- 

 peribd Glycine inax var. Biloxi and Kalanchoe blossfeldiana, which upon 

 continued growth eventually differentiate floAver primordia in short dark 

 periods. The short-dark-period plant Baeria chrysostoma (Sivori and 

 Went, 1944) maintains its photoperiod definitiveness for only a short 

 time. An extreme case of continued vegetative growth is afforded by 

 Sedum specfabile maintained on long dark periods for 9 years and by 

 S. woodwardii maintained for 8 years, both of which were brought to 

 flowering in a few weeks on short dark periods (Garner and Allard, 1931b). 



Photoperiodic control is closely associated in some animals with the 

 pituitary function, which apparently is secondary to the primary stimu- 

 lus. Hypophysectomy of the ferret (Bissonnette, 1938) and the duck 

 (Benoit and Ott, 1944) prevented gonad developnent under favorable 

 photoperiodic cycles, thus demonstrating the intermediary action of the 

 pituitary hormones. Removal of the gonads of the varying hare (Lyman, 

 1943), on the other hand, did not stop the change in coat color, nor did 

 similar treatment of crows interfere with their southward migration in 

 autumn (Rowan, 1946). Knowledge of hormonal control in arthropods 

 and other invertebrates is less developed, and the organs controlling 

 sexual function are still unknown in many cases. 



Under photoperiodic conditions that are not entirely adequate for full 

 sexual development, animals are partially stimulated. This was shown 

 to be the case for ferrets (Bissonnette, 1932a), starlings (Burger, 1949), 

 and sheep (Yeates, 1949). Whether aftereffects in the sense discussed 

 for plants occur in animals is still unknown. In many birds such as the 

 turkey, Meleagris gallopavo, the production of one or more clutches might 

 follow a photoperiodic stimulus (Scott and Payne, 1937). The after- 

 effects could be the continued functioning of the induced pituitary, but 

 the information available is too limited to attempt correlations. In ani- 

 mals the chain of events between perception and stimulation of the pitui- 

 tary or other control organ is unknown, as it is for plants. In plants 

 the organ of perception is often still enlarging, and for this reason it might 

 be more susceptible to permanent modification of functions that control 

 reproduction by suitable photoperiodic conditions than are the organs of 

 perception of animals that respond cycHcally through a multiplicity of 

 reactions in the mature organism. 



Some animals are like plants in that, although photoperiodically sensi- 

 tive, they may still become sexually active if held for long periods under 

 adverse photoperiodic conditions, as was found for ferrets by Bissonnette 

 (1932a). The potato aphid, Macrosiphum solanifolii (Shull, 1929), by 



