1899.] NATURAL SCIENCES OF PHILADELPHIA. 101 



Lactuca following regularly the spiral against the sun would have 

 the leaves back to back, when the next would twist in the opposite 

 direction so as to face the lower leaf. The occasional pairs of 

 leaves, drawn together in iDarallel lines, present a very novel and 

 interesting appearance. These cases of antidromy follow no numer- 

 ical rule. The most striking cases of parallelism will naturally 

 be those where two successive leaves in the spiral face each other ; 

 but generally it is the next in the series, or, in some cases, the 

 third or fourth leaf that turns its back on what seems to be the 

 normal spiral course. 



It may be here noted that the numerical order in the phyllotaxis 

 is not as definite as it is supposed to be, and this fact will have 

 some bearing on the phenomena connected with Lactuca Scariola. 

 In many plants which have a normal f arrangement, 4 cases are 

 not infrequently on the same stem. This may be well observed in 

 the thick- stemmed Centaureas.* The upper portion of the stem 

 thickens from a considerable distance below the flower, and the 

 numerical arrangement changes according to the degree of thicken- 

 ing. The truth is, as laid down already in these contributions, 

 the cortical layer in a growing branch is made up of the thickened 

 bases of leaves, and the elongated growth of a branch is not in a 

 straight line, but is simply the uncoiling of a growing mass of leaf 

 stalks, the terminals of which become free or proper leaves, as we 

 commonly understand them. That this is the real truth can readily 

 be perceived in many of the stronger growing Compositee. In a 

 Centaurea macrocephala, now before me, the stem can be readily 

 perceived to be formed in this manner. The lower haf in the 

 spiral laps over the one next above it like a slate or shingle on a 

 roof, and it is the width in projiortion to the length of the spiral 

 coil that decides the numerical order in the 2ihyllotaxis. The accel- 

 eration or arrestation of the uncoiling energy will therefore easily 

 explain for us these apparent aberrations from the normal condi- 

 tion. In some plants the energy born of the spiral motion does 

 not cease with the unfolding of the membranous or leaf-blade 

 portion of the structure. In many composites^ there may be two 

 or even three twists to the leaf -blade, giving it a singularly curled- 

 up character. 



* Centaurea macrocephala is especially in mind bei'e. 



® Senecio Doria and Centaurea macrocephala, for illustration. 



