PlIYLLOTAXIS OK Sl'KCri-AKlA I'KKFt >M ATA. 



V. M. Adhkws. liidiMiiii riiivorsily. 



Leaf aiTaiiiloiueiit in plants lias always hvvu a \h,\u\ ..1' li.lcrest aiA has 

 liccu llu' suhjiM't of nim-h study. Sonu! plants have a very open or loose 

 ari-aii.i;(>niont of tlioir loaves as when tlioy are separated by considerable 

 distances on the stem as in some alternate or opposite leaves. The number 

 or arrangement becomes greater and denser in other plants until their leaves 

 form rosettes or culsters as in the common HonsclcfU. Or again as in the 

 cones of the genus Pinus \vhos(> scalc-liUr loaves often form conical-like 

 cylinders of closely set divisions. Vari.ms theories were b.ng ago advanced 

 to (>xplain the arrangement (d' leace^ on the stem and especially by Ce.salpino 

 and r.onnet that the arrangement on the stem is in keeping with dehnite 

 "geometrical rules". Also many others among them A. F. Schimi)er had 

 formulated a theory on the subject. An excellent summary of many of the 

 facts on this subject has been collected by Sachs. But the effort of the 

 plant is to arrange the leaves on its stem in such a manner that they will 

 have the best exposure to the light. Also the questi(m of structural phy- 

 sical factors in the plant itself enter into the placing of leaves on the stem. 

 The arrangement which a given species shows is followed by all individuals 

 of that species although, as will be seen later, this may be departed from 

 to a certain extent. This difference I found rather markedly shown in 

 the spiral arrangement of the leaves of Specularia perfoliata. Gray* de- 

 scribes Specularia perfoliata as follows: "Somewhat hairy. 1-9. dm. high, 

 leaves roundish or ovate, clasping by the heart-shaped base, toothed, flow- 

 ers sessile, solitary or 2-3 together in the axils, only the upper or later 

 ones having a conspicuous and expanding corolla, capsule ellipsoid, short, 

 straight, opening rather below the middle: seeds lenticular". 



Nothing is said by Gray, Britton. or Wood about the rather marked and 

 regular arrangement of the leaves on the stem of this plant in spirals. This 

 arrangement together with the form of the leaves is a sti-iking characteristic 

 of Specularia perfoliata. and reminds one but to a much less degree of the 

 unusually ".si>irally twisted raceme" of flowers of Spiranthes gnicilis. 



AVhen making a trip into Brown County. Indiana, during the summer of 

 T.C'O. I noticed by the roadside about one mile from Belmont, near the studio 

 of the artist, Dr. T. C. Steele, a large number of specimens of Specularia 

 l)erfoliata. Most of these plants were of normal size and appearance. 

 Their leaves were arranged on the stem in the usual way and as to number 

 showed four to a single turn of the stalk which is the ordinary number. 

 Als(j ordinarily three circuits of the stem must be made before a leaf will 

 be found that will stand on the stem directly above the first leaf with which 

 the count was begun. In other words the twelfth leaf, counting the one at 

 the starting point, will stand directly over this first one where the spiral 

 was originally started toward the base of the stem. In this arrange- 



'Sachs, J. History of Botany 1875 P. 16S. 



=Sacks J.. History of Botany 1S75 P. 162. 



"Saclis, .T. History of oBtany 187.">, Chaptor 4, PP. ^.^.^-l<Sl. 



^Gray, A. Now Manual of Botany, 7th edition. 



