Roberts and Struckmeyer — 93 — Anat. and Histol. Changes 



observed in all of 65 to 70 species of dicotyledons which have been ex- 

 amined. 



The time and nature of the transition from the anatomical condition 

 which is typical of non-flowering plants to that found in the flowering 

 stems has been determined and described (34). A marked reduction in 

 cambial activity and a corresponding increase in maturation of xylem and 

 phloem elements was clearly apparent in stems of Salvia splejtdens var. 

 Harbinger in 5 days after placing the plants in an environment inducive 

 to flowering (short days and a warm night temperature). This is three 

 to four days prior to the appearance of blossom primordia. 



Different species exhibit different rates of reaction to an environment 

 favorable to blossom induction. At optimum photoperiod and temperature, 

 soybeans {Glycine max var. Biloxi, a short-day type) showed marked 

 anatomical changes in five to six days and blossom primordia in nine to 

 twelve days. Xanthium echinattmr responds more rapidly ; anatomical 

 changes were obvious in three days and blossom primordia were found in 

 four to five days after the start of short photoperiod treatments. Cosmos 

 sulphureus var. Klondike showed very distinct changes in anatomical struc- 

 ture after three short days and blossom primordia were observed in twelve 

 days. 



In the long-day species Matthiola incana (Stock, var. Christmas Pink) 

 blossom primordia were found after 17 to 18 long light periods. Changes 

 in anatomical characters were found in these plants after only five to six 

 days of treatment. 



From the preceding paragraphs it is evident that the first appearance 

 of blossom primordia would not be an index of when induction begins. 

 In fact, in the apple (38) and cranberry (31) induction has progressed 

 to a stage where nearly complete defoliation will not interrupt it, as long 

 as three to four weeks before primordia arise. This would seem to bring 

 into question the desirability of dissecting the tips of plants to observe the 

 appearance of primordia as is insisted upon by some workers. Changes 

 in anatomical structure appear to correspond very closely to induction. 

 Observation of this situation may be found to be useful if it is desired that 

 the time of initiation be determined accurately. 



The length of photoperiodic treatment needed sufficiently to induce a 

 plant so that it becomes sexually reproductive even after being transferred 

 to an environment inhibitory to induction also varies with the species or 

 variety. Biloxi soybeans need approximately 17 short days (34) to estab- 

 lish effective "after-affects." It will be reported later that plants which 

 flower terminally as Salvia splendens var. Harbinger will return to a vege- 

 tative type of growth even well after they are in flower if the plants are 

 given long-day conditions. That is, they do not exhibit fixed after-affects. 



It is reported (7) that Xanthium becomes sexually reproductive after 

 only one long dark period (one short day) even though returned and con- 

 tinued in long days. Staminate flowers will usually but not always be 

 produced by plants given one long dark period but in repeated trials it has 



^ From cultural comparisons and reactions to photoperiod this is quite obviously 

 the same species as that used by Hamner and Bonner (7) and called A', pcnnsyl- 

 vanicum by them. 



