Tyl oses in Tracheids of Conifers. 203 
Sequoia , 1 and he considers this phenomenon a reversion to the 
ancestral condition in which resin canals are normally present, as 
in the group Pineae. There seems good reason for considering 
resin canals to be a feature of phylogenetic importance ; are tyloses 
to be so regarded ? According to the observations recorded in this 
paper, tyloses occur only in Pinus, which appears from the 
researches of Jeffrey and others to be a very old genus, but this 
does not necessarily imply that tyloses are a primitive feature. It 
is of interest to note in this connection that Conwentz 2 has reported 
the occurrence of tylosis-like growths in the tracheids of the amber 
producing conifers of the late Eocene, which were pines, though his 
observations have been called in question by Raatz. Moreover, 
tyloses have recently been described by Weiss (he.) as occurring in 
the tracheids of a fossil fern Rachiopteris, from the coal measures, 
so that they may fairly be regarded as an ancient structure. The 
occurrence of tyloses in the cone axis seems to be a significant fact, 
for the reproductive axis is a region where ancestral features are 
prone to persist. It may also be remarked that roots have a more 
primitive structure than stems, and it is in the roots that tyloses 
are most abundant, at least in the conifers. But the failure to 
produce these growths by means of wounding in other genera than 
Pinus does not argue in favour of their being ancestral structures. 
Perhaps the production of tyloses is to be explained on purely 
physiological grounds, as is indicated by the experiments on wounded 
roots. But it is still unexplained why tyloses frequently occur in 
roots and seldom in stems of pines. Raatz’s suggestion that roots 
are more subject to injury by tramping, etc., fails to account for 
the prevalence of tyloses in heart wood. It is well known that in 
the genus Pinus it is only in the heart wood that tyloses fill the resin 
canals, and it may be argued that production of these growths in 
both resin canals and tracheids is involved in the transformation of 
sap wood into heart wood, and that in both cases the tyloses serve 
to check the entrance and progress of fungal hyphse. It has been 
suggested that the “ Eiporen ” in the walls of the medullary ray 
cells of Pinus are especially favourable for the protrusion of a 
tylosis, but this explanation loses its force when we consider the 
fact that in Quercus alba and Robinia pseudacacia the pits on the 
1 Jeffrey, E. C. The Comparative Anatomy and Phylogeny of 
the Coniferales, Parti. The genus Sequoia. Mem. Boston 
Soc. of Nat. Hist. 5 ; X., pp. 441 — 459, Pis. 68—71, 1903. 
1 Conwentz, H. Ueber Thyllen und Thyllenilhnlichen Bildungen. 
Berichte d. deut. bot. Ges. VII., pp. 34—40, 1889. 
