Hill. — On the Presence of a Parichnos in Recent Plants . 271 
This is a result which might have been expected on account of the relative 
smallness of the leaves. Phylloglossum also has given negative results. 
The reasons for considering these canals as representing essentially the 
same structure as the parichnos of fossil plants are sufficiently obvious, and 
may be briefly enumerated : — 
1. Both in recent and fossil plants they occupy the same relative 
position. On the one hand there is a double strand with the vascular 
bundle between and slightly above, as in Lepidophloios (Fig. 14) and 
Isoetes (Fig. 15) ; on the other, a single strand beneath the vascular bundle, 
as in the case of Lepidostrobus and Lycopodium (Figs. 12 and 13). 
2. The appearance in section is frequently identical. The hollow 
canals of Lepidocarpon , for instance, may be compared with the mature 
structure in Isoetes . The parenchymatous strand in Lepidodendron is 
similar to some of the early stages in the development of the canals in 
Isoetes and Lycopodium . Thus Figs. 16 and 18, representing the parichnos 
of Lepidophloios and Lepidodendron respectively, may be compared with 
Figs. 15 and 17, illustrating a stage in the development of the mucilage-duct 
in Isoetes Hystrix. 
The main point of dissimilarity lies in the fact that in Isoetes the 
parichnos does not extend into the cortex. 
This may be considered an unimportant matter — that so ancient 
a feature should be altered somewhat in recent plants is to be expected, 
the surprising thing would be to find the tissue entirely identical both in 
recent and fossil plants. 
Function. 
It is manifestly a matter of no inconsiderable difficulty to assign 
a definite function to a tissue of so specialized a nature as the parichnos 
occurring in fossil plants. Consequently, it is not surprising to find that in 
this respect the authorities differ. Hovelacque considered the tissue as 
being glandular, with which view both Williamson and Bertrand disagreed. 
Renault regarded the tissue as representing gum-canals, Potonie as 
transpiratory organs. Scott 1 i while admitting the possibility of the 
secretory function, points out that ‘ the persistence and enlargement of 
the parichnos on the surface of old stems suggests a respiratory function 
like that of lenticels.’ This view is also held by Weiss 2 , who suggests that 
the parichnos was probably c a respiratory organ, allowing a passage of air 
from the leaves into the inner portion of the stem and down into the roots.’ 
He compares the structure with ‘ the trabecular tissue of various species of 
Selaginella , which also accompanies the leaf traces, and probably has the 
same function as the middle cortex and the parichnos, but the persistence 
of the parichnos on the leaf scars of the old stems of Lepidodendra enabled 
1 Scott, Studies in Fossil Botany, London, 1900. 
a Loc. cit. 
