178 PROCEEDINGS OF THE ACADEMY OF [1897. 



flowers, with absolutely no function to perforin, present an anom- 

 aly, and yet it has a counterpart in the petal-bearing flowers of 

 many cleistogamic species, which I have found rarely seed-produc- 

 ing, though it is customary to refer this class of flowers, in such 

 cases, to an effort on the part of the species to secure an occasional 

 cross. It would rather seem that the true position of cleistogamy in 

 the economy of nature is not yet well understood. 



RHYTHMIC GROWTH IN PLANTS. 



Though the principle that plant-growth is not continuous but 

 rhythmic must have been long ago observed, 1 am not aware that 

 any special importance has been attached to it, or of any detailed 

 observations as to the time and manner of the rests and advances 

 until my paper on the Compass Plant, Silphium laciniatum, ap- 

 peared in the Proceedings of the Academy in 1870. I believe 

 it has been left wholly to me to show that rhythmic growth is 

 an important factor in the evolution of form. It is not even yet 

 recognized as it deserves to be. This consideration renders the 

 recording of additional facts desirable. 



Dr. Asa Gray, in the Synoptical Flora of North America, refer- 

 ring to the natural order Polernoniacese, says, " hypogynous disk 

 generally manifest," and " remarkable among the hypogynous gamo- 

 petalous orders for the trimerous pistil, but in two or three spe- 

 cies of Gilia dimerous." " The corolla is not always perfectly 

 regular, and the live stamens are very commonly unequal in length 

 or insertion." 



Of Phlox particularly he says, " most species with long filiform 

 style, about equalling or surpassing the corolla tube, but some 

 with short included style, perhaps but dimorphous ; but only in P. 

 subulata have both forms been found in the same species." 



Unless we remember the rhythmic character of growth, we get but 

 a poor idea of the true nature of things by the terms Dr. Gray em- 

 ploys. What do we learn of the disk by merely calling it by this 

 name ; and what is a dimerous or a trimerous pistil ? Why should 

 the stamens and style be dimorphous, and what is dimorphism in 

 this connection? Before answering these questions, let us follow 

 the development of the flowers in Phlox. 



Early in the morning of September 1st, all the expanded blossoms 

 of some heads of garden varieties of Phlox paniculata were plucked 

 out so as to watch the behavior of the unexpanded flowers. As I 



