GENERAL MORPHOLOGY 431 
EXTERNAL CHARACTERS. 
The plants of this cosmopolitan family are all perennials, and are for 
the most part underground-growing organisms, though some few are 
epiphytic. The method of their perennation is closely connected with 
their external form. Given a leafy shoot in an underground-growing 
organism, there are two ways in which it may be specialised so as to 
secure perennation, and often the arrangements are such as to fit in 
conveniently with alternating seasonal periods of activity and of rest. The 
one is by elongation of the internodes, accompaniéd by repeated branching : 
in this case the terminals of certain branches themselves appear above 
ground in the active season, and may die off at its close, the perennation 
being effected by the branching stock which remains in the soil: such 
shoots are usually small-leaved, and examples are seen in Eguisetum, and 
in some of the more specialised species of Lycopodium and Selaginella, 
where the primitive upright habit of the main shoot has been discarded. 
The other method is by enlargement of the individual leaf, while the stock, 
which is sparsely branched or even unbranched, remains protected below: 
this is exemplified by Jsoe¢es in a less pronounced form, but in its most 
extreme type by the Ophioglossaceae, and by some Ferns of such habit 
as Pterts aguilina. The stock itself in such plants is provided with 
sufficient storage-tissue, and may in some species be specially distended 
and tuberous (O. crotalophoroides, Walt., and O. opacum, Carmich.). This 
type tends to become monophyllous, with only one large leaf expanded 
in each season. The chief biological advantage in the monophyllous 
habit in a plant with a perennial stock lies in the fact that the soil 
presents an obstacle to the upgrowth of the tender young leaf: the difficulty 
of overcoming this is minimised by the production of only one leaf in 
each season, and that a large one. This would apply equally to the -case 
of Pteris, and to that of the Ophioglossaceae. 
It is then as organisms showing a peculiar specialisation for a perennating 
habit that the Ophioglossaceae are to be studied. There is one further 
point on which it is necessary to be clear at the outset: the Lycopods 
and the Horse-tails are small-leaved forms and show a similar method of 
perennation: but still they are held to represent distinct phyla. Similarly, 
though the Ophioglossaceae and the Ferns may show in common another 
mode of perennation, accompanied by large foliar development, still this 
does not in itself indicate any near relationship: for clearly leaf-enlargement 
is not the prerogative of one phylum only. 
Taking first the genus Ophioglossum, the well-known species O. vulgatum 
occupies a middle position in the genus (Fig. 235): it consists of a short 
upright stock, covered externally by the scars of leaves expanded in previous 
years: thick roots, which are commonly unbranched (though occasionally 
showing dichotomy), and hairless, radiate from it, one being inserted as 
a rule below the base of each scar; but this arrangement is not rigidly 
