Age and Flowering in Herbaceous Plants • 119 



discussed earlier, the discussion below will be concerned primarily 

 with other prerequisites for the photoperiodic control of flowering. 



Klebs (1918) originated this field of inquiry by observing that 

 Sempervivum funkii did not show a flowering response to long 

 days until it had been growing for some time, and he concluded 

 that the best conditions to bring about this Bliihreife state were 

 those involving a high degree of carbon dioxide assimilation and 

 a relatively meager mineral nutrition. This, as well as other obser- 

 vations by Klebs, was in part the origin of investigations on the 

 C/N ratio (Chapter Six). It seems clear now that for most photo- 

 periodic plants, probably including Sempervivum, gross nutrition 

 is less important than the morphological stage of development 

 attained. 



Certain plants do not respond to an inductive photoperiod 

 until they have produced true leaves, but there are some in which 

 the cotyledons themselves are sensitive. These include the SDP 

 Pharbitis (Nakayama, 1958) and Chenopodium rubrum, some 

 strains of which may flower as tiny seedlings barely emerged from 

 the seed coat (Cumming, 1959; see illustration facing page 1). The 

 SDP Xanthium and Perilla, on the other hand, are of the former 

 type. The development of at least one true leaf is necessary before 

 Xanthium can respond to short days. Jennings and Zuck (1954), 

 testing the possibility that this might be due to insufficient area of 

 the expanded cotyledons, found that an area of true leaf consid : 

 erably smaller than the total cotyledon area could induce flowering. 



In Perilla, the sensitivity to induction Of successive pairs of 

 leaves increases from the second to at least the fifth pair, with the 

 first and second being almost insensitive. This again does not appear 

 to be a matter of leaf area or even of plant size, but represents 

 a developmental difference in the leaves. For example, if equal areas 

 (see Fig. 5-3, p. 86) are cut from second and fifth leaves, grafted 

 onto other plants in long day, and then induced with short-day 

 treatments so that they will function as donors, the tissue from the 

 fifth leaves is by far more effectived However, the fact that intact 

 older plants respond more quickly than younger plants is also due 

 to greater total leaf area (Zeevaart, 1958). In the grass Lolium 

 temulentum, the increasing sensitivity of the entire plant to photo- 

 period is attributable entirely to the increasing sensitivity of suc- 

 cessively produced leaves. When only several lower leaves are left 



