Vascular System of Matonia pectinata. 
503 
Protoxylems. 
The question of the morphological value of protoxylems is one of 
considerable interest. There is no cfoubt that in many cases at least 
the centres of differentiation of the xylem (Bertrands poles tracheens) are of 
morphological importance, affording a kind of basis, or skeleton, on which 
the whole xylem-system is built up. Not only is this clearly the case 
in the roots of most vascular plants, where the position and often the 
number of protoxylems within considerable circles of affinity is very con- 
stant, but it is certainly also the case in the vascular strands of the shoots 
of many groups. We need only refer to the universal exarchy of the steles 
of Sphenophyllales and Lycopodiales, and to the endarchy of the Angio- 
sperms, or to Scott’s important demonstration of the gradual transition 
from exarchy to mesarchy in the leaf-trace, and to endarchy in the stele, of 
the great Cycadofilicinean series, in order to emphasize the point. 
In the Filicales proper the case is rather different. The exact position 
of the spiral protoxylems in relation to the metaxylem of the vascular 
strands of the stem is decidedly variable. Exarchy, endarchy, and mes- 
archy are all found within comparatively small groups, and the actual 
course of evolution seems to affect the position of the protoxylems much 
more freely and rapidly than in the other great groups of vascular plants. 
Furthermore, as the researches of Boodle showed in Hymenophyllaceae 1 
and Schizaeaceae 2 and those of Gwynne-Vaughan in Loxsoma 3 and various 
Davalliaceae 4 , the spiral protoxylems of many Ferns are confined to the 
leaf and are absent from the stem altogether. This phenomenon, as Boodle 
showed, is connected with the slow growth in length of the stems of the 
Ferns in question, and in Trichomanes and Dicksonia at least we get the 
presence and absence of spiral protoxylem in the stem in different species 
of the same genus. It is in fact quite likely, though this has never been 
definitely established, that spiral protoxylem is not formed except where 
definite growth in length occurs after the differentiation of the first formed 
tracheal strands. Moreover, spiral protoxylems in the stems of Ferns 
are always, according to Gwynne-Vaughan 5 , and so far as his observations 
extend, continuous with those of the petiole. In those cases in which there 
is no spiral protoxylem in the stem there may be a localized non-spiral 
protoxylem-band round the stele, which is exarch ( Loxsoma , various 
Davallias, & c.) or endarch ( Schizaea malaccana 6 ), but which has no con- 
nexion with the leaf protoxylem ; or the differentiation of tracheids may 
be more or less irregular ( Gymnogr amine , Lindsay a, &c.). In Dicksonia 
we have in the stems of different species the three cases— endarch or 
1 Boodle (’00). 2 Boodle (’01 A). 3 Gwynne-Vaughan (’01), p. 79, and (’03), p. 727. 
♦ Gwynne-Vaughan (’03), p. 728. 5 1. c. (’03), 6 Tansley and Chick (’03), p. 508. 
