26 ME. C. E. JONES ON THE MORPHOLOGY 
portion of the stem bears reduced leaves; these become larger and more numerous till 
the ordinary condition of the young plant is reached about 3 inches above the prothallus. 
A plant as young as this seems to show a truly radial facies and is orthotropous. The 
other plant (B) is probably a good deal older, but not much longer. This differs from 
the previous plant, since at a certain point the growth has been modified and the 
structure is suddenly altered. It is possible that this break represents the interruption 
of growth at the end of the season. This obvious interruption was evident in all but the 
youngest plants, and in certain cases is clearly shown to occur where a branch arises. 
It is a possibility that the interruption is due to the transfer of activity from the stem 
to a branch, when the young plant would present a sympodium and not a monopodium; 
but this would not be in keeping with the ordinary process of branching which obtains 
in the species. It will be observed that the principal root in B springs from the stem 
at a much higher point. The more dorsiventral the shoots the more widely separated 
are the roots, and in the case of erect stems they are clustered towards the base. The 
principal feature observed in sections of the young stem is that the vascular bundles 
present a triarch or tetrarch arrangement (P). 5. fig. 29), exactly similar to the structure 
which may be found in the roots of Gymnosperms or Dicotyledons: the xylem forms a 
central xylem-plate, so that the four phloem-portions are separated; the pericycle is one- to 
three-layered and the endodermis shows one or two rows of cells. The thick-walled cortex 
extends through four rows, and lies against thin-walled tissue which is easily broken down; 
beyond this a zone, about three cells deep, adjoins the epidermis, which is provided with 
a well-marked stratified cuticle. The young stem presents, therefore, exactly a similar 
arrangement of cortex to that which is found in the adult stem. So far as the stem 
was followed downwards a triarch or tetrarch structure was maintained, but it is not 
impossible that this might be modified into a diarch structure. A section which was 
shown to me, labelled “first root," showed what might be regarded as a diarch or 
tetrarch structure, in which the phloem occupied the central position and separated the 
two xylem-patches. Such a structure is not uncommon in the adventitious roots, where 
it is easily explained; but if this obtains in the first root there would be very good 
reason to look upon it as a late development from the stem, and notas primary. Passing 
upwards it will be found that, owing to changes which should be considered in reference 
to the apical meristem, the protoxylem extends along the periphery (Pl. 5. fig. 31), so 
that it overlaps the phloem, and eventually the far end connects up the central xylem- 
plate and the phloem becomes isolated (fig. 32). 
case of the same or another protoxylem, and thus 
pentarch and then to hexarch (figs. 33 & 34). 
: Similarly, seven, eight, or more proto- 
xylems are differentiated. This process 
is not only to be observed in the young stem, 
adult stem are traced up, there is found to be 
In this way, if a long shoot is being formed, 
say, upon a stem showing twelve protoxylems, this may drop to ten, or fewer, in the 
branch Which arises, and then by the splitting of a protoxylem eleven are formed, and 
this repeated leads to the same number as the original stem possessed. It generally 
The same process occurs again in the 
the structure changes from tetrarch to 
the protoxylems of the stem, in many cases 
