1912] HOLDEN—SAPIN DALES 55 
descended from ancestors which had multiseriate rays like those of 
Acer, Sapindus, and Siaphylea. In determining this point, one 
must rely on the principles of comparative anatomy worked out for 
the gymnosperms, and investigate the parts of a plant which are 
most tenacious of ancestral characteristics, namely, leaf petiole, 
reproductive axis, and root. 
Figs. 7 and 8 represent transverse sections of the leaf petiole of 
Aesculus Hippocastanum; fig. 9 a tangential section of the same, 
and in all three the multiseriate type of ray is conspicuous. Whena 
leaf petiole leaves the branch, there is no one woody cylinder, but 
instead, 20-30 small vascular strands. Most of these strands 
arrange themselves in the form of a circle, and fuse to form a sipho- 
nostele, but certain ones, perhaps 5-10, instead of taking up a 
peripheral position, remainin the center. These medullary bundles 
are at first collateral in structure, but soon the xylem begins to 
grow around the phloem, until they become amphivasal, forming 
bundles such as are found typically in monocot rhizomes. This 
siphonostelic condition with medullary bundles is found throughout 
the length of the petiole, up to the bases of the leaflets. Then the 
cylinder is broken again into a large number of vascular strands, 
which in the bases of the leaflets repeat the process carried on in the 
base of the petiole. Some take up a peripheral position and form 
a siphonostele, while one or two remain in the pith as medullary 
bundles. These medullary bundles, however, are always collateral, 
never amphivasal. The important point is that throughout the 
prevailing type of ray is multiseriate. This is equally true of the 
separate strands as they leave the main branch, of the woody 
tissue of the siphonostele of both petiole and leaflet, and of the 
medullary bundles of both petiole and leaflet. Usually the rays 
as they leave the pith are biseriate or triseriate; sometimes they 
remain so to the cambium, but usually they become reduced to a 
uniseriate condition. Another peculiar condition seen in tangential 
section is the longitudinal elongation of the ray cells. 
It is one of the principles of plant anatomy that the leaf trace 
is tenacious of ancestral conditions, and it is interesting to note 
that in the case of Aesculus these primitive conditions are retained, 
