AND ANATOMY OF THE STEM OF LYCOPODIUM. 27 
happens that as soon as six or seven protoxylems are formed, either because more phloem 
is required than can lie in the bays or because sufficient xylem is not formed to keep the 
central portion intact, two phloems become connected across (fig. 34), and thus the banded 
structure originates which characterises the clavatum group. After the stem has attained 
a complicated structure, then in the branches, more especially those arising on the short 
shoots or branches of the second or higher orders generally, owing to the small number 
of protoxylems which pass off, the structure is reduced, until, in branches which have 
only six or fewer protoxylems, these are again connected up in the centre by metaxylem 
and the original triarch or tetrarch structure is attained ; this must be regarded as a 
reduced structure, as in the case of L. inundatum (Pl. 4 fig. 20). It is doubtful 
whether in any seedling the protoxylems have such a broad periphery as shown in this 
figure. This explanation, it will be observed, does not lend any support to the gamostelic 
nature of the structure of the vascular cylinder which has been postulated, but is of the 
nature of a gamodesmy in which the strands which pursue a sinuous course are constantly 
dividing and reuniting. 
Young plants of Lycoroprum SELAGO, Linn., and LYCOPODIUM SERRATUM, Thunb. 
Failing any other young plants derived from the prothallus which would furnish 
comparative evidence, the young plants derived from bulbils were examined. The 
external morphology of these bulbils has been described by Hegelmaier (5) and 
Strasburger (8); but these authors do not discuss the vascular structure, except to note 
that a small bundle, the equivalent of a leaf-trace, passes upwards, and the traces pass off 
from it. Starting from the base of a young plant, to which the bulbil was attached, the 
original simple bundle was found. This contains a single xylem-mass, which spreads 
out and separates into two portions, leaving a phloem-band in the middle (Pl. 5. fig. 35); 
this diarch stage is the commonest one to be found at the base of the stems of the young 
plants both of L. Selago and serratum. It is a structure which occurs fairly commonly 
also in roots, both where these run through the stem and also in small external roots or 
rootlets. The protoxylems can generally be distinguished on the outside of the periphery 
of the xylem in a median position. The phloem-tissue is so homogeneous that it is not 
possible to say whether it should be regarded as made up of two portions or one. By the 
extension and subsequent division of one of the two xylems a triarch structure is 
developed. In this case, generally the xylems are still separated, and this may persist 
after a tetrarch condition is reached. 
In these young plants growing from bulbils the origin of the vascular cylinder is 
different, in so far that it arises from a leaf-trace, and also since the phloem forms the 
central portion ; but the method of increasing the protoxylems is exactly similar to that 
described for the seedling of L. clavatum. | 
There seems, then, to be no doubt that the vascular cylinder in the stem of Lycopods 
has started from a structure which is exactly similar to that found in roots at the 
present day, and also there is the same formation of thick-walled cortex immediately 
surrounding it. It may be presumed, as it has always been suggested, that it represents 
