479 
Vascular System of Matonia pectinata . 
main fork in the adult. In the angle of the dichotomy, however, a 
‘middle lobe’ is developed in all but some of the simplest fronds, just 
as is often the case, for instance, in the branching of the thallus of the 
Liverworts. It is this middle lobe which gives to so many of the fronds 
of the young plants their trifoliate character (Figs. 2, 6, and 7). In the 
adult the middle lobe is represented by the median pinna k The subse- 
quent branching of the primary forks and of the middle lobe at first 
(i.e. in the young plants) shows a tendency to be dichotomous, but the 
dichotomous systems easily pass over into the monopodial type, and the 
three lobes of the trifoliate leaf are in most cases pinnately branched and 
correspond with the 6 pinnae ’ of the adult. Some of the starved plants 
have trifoliate fronds whose three lobes resemble in all respects the ‘ pinnae ’ 
of the adult frond. 
The origin of the adult frond from this type of structure is not 
at first sight quite obvious. We suggest that it arises by the dichotomy 
of the primary forks and the repeated dichotomy of the loiver member only of 
each successive fork, the upper member in each case, and the lower member 
also of the last fork, becoming a pinna. If this be so, the ‘ recurved arm ’ 
is a sympodial axis composed of the bases of the lower members of 
successive dichotomies. To suppose that it is a monopodial axis giving 
off lateral members from the upper face only would be to assume a type 
of branching unparalleled, so far as we know, in the fronds of Ferns, 
while the ‘ scorpioid ’ character of each half-frond certainly suggests 
a sympodium. Furthermore, the hypothesis of dichotomy as the funda- 
mental type of branching in the frond as a whole, enables us to bring 
it into relation with the fronds of Gleichenia. Many of the species 
of this genus, as is well known, have long straggling fronds with repeated 
dichotomous branching. ‘ Pinnae ’ may be borne on the primary forks, 
or may be confined to those of a higher order. A bud normally arises 
from the angle of the primary dichotomy. This bud is sometimes 
developed, forming a main rachis of the whole frond, and itself dichoto- 
mizing and bearing pinnae to a greater or less extent. Boodle (’01 B, 
p. 705) has already suggested that it is possible to derive the pinnate 
type of frond, so common among the Ferns, from such a dichotomous frond 
as is found in these straggling Gleichenias , by imagining the successive 
forks of this main rachis to be limited in growth so that they become 
pinnae. Such a mode of origin is all the more likely since some species 
1 It is possible to regard the ‘ middle lobe ’ not as a structure sui generis , so to speak, as it 
apparently is in the thalloid Liverworts, but rather as in origin the inner branch of the second 
dichotomy, becoming in the adult a pinna which, like the other inner branches, has lost the power of 
further dichotomy. Such an interpretation is indeed suggested by Figs, i, 2, and 5, and is perfectly 
compatible with the structure of the adult frond. No light is thrown upon this question by any of 
the adult fronds of Gleichenia we have examined {vide infra ), though an extended comparative 
examination of the fronds of^w«^Gleicheniaceous plants might be expected to illuminate the whole 
subject. 
