i«7 
Xylems in the Petioles of Cycads. 
show the facility with which bundles or steles can adapt themselves to 
either morphological or physiological requirements. Their main significance 
is that they illustrate the need of caution in basing phylogenetic relation- 
ships on morphological structures which may be the result of long and 
progressive efforts of the plant to adapt itself to different needs. 
More worthy of notice is the special and constant arrangement of the 
elements themselves, with their regular rows of xylem, cambium, and 
phloem, separated by distinct medullary rays (Figs, i and 5). Such 
a structure would naturally suggest a secondary growth, an assumption 
made by several writers with less definite or clear sections before them, at 
least judging from the sketches they have published. 1 I was too ready to 
make the same assumption when Professor Lang, of Manchester, warned 
me that there are similar occurrences in primary structures. A section 
across a petiole of Ginkgo , for instance, would show it clearly. 2 Such 
a structure may be a predetermined arrangement and not the result ot 
cambial activity. This objection is a most serious one, attacking the very 
foundation of the argument and, consequently, the conclusion itself. We 
must therefore recast the whole proof or give up the assertion. 
It is a difficult problem to work out critically owing to the impossibility 
of studying the same individual bundle at different ages ; on the other 
hand, the structure of the bundle varies considerably and rapidly with the 
distance from the stem or even at the same level on the petiole. 
There is, however, a possible solution. Taking petioles of different 
ages from the same species and the same stem, which have had the same 
chances and the same opportunities for growth, the only difference being 
that of age, we may count the number of cells contained in a row passing 
straight from the cambium to the first protoxylem cell. To be fair we 
must take for comparison the highest number of cells found in a row at 
different ages in different petioles, or we might also use averages ; the 
former method was followed. 
The present account is based mainly on Cycas revoluta and Cycas 
circinalis . The youngest petioles used were about 10 cm. long. Dr. Carano 
gives a sketch of a still younger specimen. 3 His drawing would seem to 
lend a strong support to my argument, but it is safer not to use it as we do 
not know the age of the stem from which the petiole was taken. 
The oldest petiole (probably four years) of Cycas circinalis examined 
showed straight rows of 22 lignified cells, an average of 16-20 being 
of common occurrence (Fig. 1). A specimen two years old numbered 
16 cells.. Other petioles of the present year showed a decrease corre- 
sponding with their age, the maximum being about 12 in a specimen 
40 cm. long. A young leaf of Cycas revoluta , 10 cm. long, possessed rows 
1 E. Carano (’03), and C. Eg. Bertrand et B. Renault (’ 86 ). 
2 A. Sprecher (’07), p. 64 . 3 E. Carano (’03), Tav. ix, Fig. 9 . 
