GENERAL MORPHOLOGY 431 



X 



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 

 icure perennation, and often the arrangements are such as to fit in 

 mveniently with alternating seasonal periods of activity and of rest. The 

 is by elongation of the internodes, accompanied by repeated branching : 

 this case the terminals of certain branches themselves appear above 

 >und in the active season, and may die off at its close, the perennation 

 sing effected by the branching stock which remains in the soil : such 

 loots are usually small-leaved, and examples are seen in Equisetum, and 

 some of the more specialised species of Lycopodium and Selaginella, 

 /here the primitive upright habit of the main shoot has been discarded, 

 ic other method is by enlargement of the individual leaf, while the stock, 

 rhich is sparsely branched or even unbranched, remains protected below : 

 lis is exemplified by Isoetes in a less pronounced form, but in its most 

 rtreme type by the Ophioglossaceae, and by some Ferns of such habit 

 Pteris aquilina. The stock itself in such plants is provided with 

 ifficient storage-tissue, and may in some species be specially distended 

 id tuberous (O. crotalophoroides, Walt., and O. opacum, Carmich.). This 

 >e tends to become monophyllous, with only one large leaf expanded 

 each season. The chief biological advantage in the monophyllous 

 ibit in a plant with a perennial stock lies in the fact that the soil 

 ;nts an obstacle to the upgrowth of the tender young leaf : the difficulty 

 overcoming this is minimised by the production of only one leaf in 

 ich season, and that a large one. This would apply equally to the case 

 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 



