The fern spoken of by Dr Wallace as Dipteris Horsfieldii (perhaps better known as Dipteris conjugata ([Fig. 12])), which grows with Matonia pectinata on Mount Ophir and in the Malay region generally, is one of seven species of a genus characterised by a somewhat wider geographical range than Matonia. Dipteris conjugata extends to the Philippines, Samoa, Fiji, New Caledonia, New Guinea and Central China; its fronds, like those of Matonia, are borne on long slender stalks attached to a creeping stem; they have a broad lamina divided by a deep median sinus into two symmetrical halves and each half is cut up into segments with a saw-like edge. Several stout ribs spread through the lamina from the apex of the long stalk like the open fingers of a hand; from these ribs smaller veins are given off at a wide angle, and these in turn give rise to a reticulum of finer veins forming a skeletal system like that in the leaves of an oak and many other flowering plants.
Numerous groups of spore-capsules are borne on the lower surface of the broad lobed frond. The leaves of other species of Dipteris have the same type of structure, but in some the segmentation of the lamina is carried further and the leaf consists of numerous long and narrow segments with one or two main ribs. Dipteris is represented in the flora of Assam, and it is interesting to find that a species recently discovered in Borneo is more closely connected with the Assam type than with those of the Malay region. Until a few years ago the genus Dipteris was included in the large family Polypodiaceae of which nearly all our British ferns are members, but the discovery of certain distinguishing features in the structure of the sporangia showed that these Eastern and Southern species form a fairly well-defined group worthy of family rank.
In the Rhaetic plant-beds of Northern and Central Europe, of North America, Tonkin, and elsewhere, numerous fossil leaves have been discovered which in shape, venation, and in the manner of occurrence of the sporangia bear a close resemblance to species of Dipteris. Ferns of this type were abundant in the Jurassic floras of the northern hemisphere, and it is interesting to find impressions of Dipteris-like leaves both in the Jurassic rocks of the Yorkshire coast as well as in slightly newer beds of the same geological period on the north-east coast of Sutherland.
It is impossible to say with confidence how nearly these Rhaetic and Jurassic ferns were related to the existing species, as our knowledge of them is less complete than in the case of the fossil representatives of the Matonineae, but there can be no reasonable doubt that in Dipteris as in Matonia we have a connecting link between the present and a remote past.
[CHAPTER VI]
THE REDWOOD AND MAMMOTH TREES OF CALIFORNIA
'Your sense is sealed, or you should hear them tell
The tale of their dim life, with all
Its compost of experience....'
W. E. Henley.