Veraalization and Photoperiodism — 92 — A Symposium 



observations that sour cherry (Prunus cerasus) (17), plum (Prunus nigra 

 Hort.) (19) and apple {Mains malus) (20, 21) have a greater stem diame- 

 ter when blossom buds are formed. Later observations (38) on the time 

 of apple blossom bud induction disclosed that the marked secondary thicken- 

 ing which is typical of "fruit" spurs is produced after blossom bud initiation 

 has begun. The anatomical differences which are characteristic of non- 

 flowering and flowering stems of the apple are like those previously found 

 in annual and biennial plants (33, 2)7) : "The flowering stems of all the 

 species examined (at the fourth internode from the stem tip) seem to have 

 certain anatomical characteristics in common, regardless of age or of photo- 

 periodic classification. In contrast to the non-flowering stem, the flower- 

 ing stem is characterized by: (i) a less active cambium; (2) a zone con- 

 sisting mainly of thick-walled secondary xylem elements lying adjacent 

 to the cambium in contrast to rather numerous vessels and thin-walled 

 parenchymatous cells in the last formed xylem of the non-flowering stems ; 

 (i) generally thicker walls of the cells of the pericycle, perimedullary zone, 

 and phloem; {4) freer staining with 'basic' dyes of the pericycle, perimedul- 

 lary zone, and certain elements of the xylem." 



Typical changes in the structure of the phloem as the reproductive state 

 is entered were also found (33) : "Some of the phloem characteristics 

 which have been seen to accompany blossoming are: (J) Limited or 

 slight formation of phloem cells following reduced cambial activity which 

 precedes blossoming; (2) Small size of later formed cells; (J) Increase in 

 cell wall thickness; {4) Increase in callose formation on sieve plates and 

 fields; (5) Accumulation of inclusions in some cells; (d) Mechanical com- 

 pression." 



Additional observations upon the relation of cambial activity to the 

 reproductive condition of some dicotyledonous plants (39) led to the fol- 

 lowing summary statements : "Vigorously vegetative plants have an active 

 cambium throughout the length of their stems. The cessation or decrease 

 of cambial activity which accompanies the production of flowers progresses 

 from the region of the inflorescence toward the base of the plant, which it 

 may or may not reach depending upon the degree of reproductiveness which 

 the plant attains as measured by the relative number of primordia which 

 differentiate as floral structures. If certain species of plants are allowed 

 to reach an advanced stage of reproductiveness under favorable environ- 

 mental conditions, the meristematic tissue of their stems tends to become 

 entirely differentiated into xylem and phloem elements. This anatomical 

 condition is a possible explanation of the death of such plants at the close 

 of one reproductive cycle." Wilton had ample evidence to have proposed 

 that the annual habit is due to a cession of cambial activity rather than 

 to have only suggested this possibility. 



Unpublished work upon the cambial condition in roots^ has shown that 

 the cambium becomes inactive or "lost" in maturing annual plants and 

 fruiting biennials and that it persists in perennials and in those annual or 

 biennial plants which have been kept in an environment which maintained 

 them in a non-flowering state. 



The relation of cambial activity and sexual reproduction has been 



1 Paul Bernstein, Lois Thomson Foster, Dr. B. E. Struckmeyer. 



