THE HIGHER CRYPTOGAMIA. 207 



the same thing also has been noticed in Salvinia natans 

 and Pilularia globulifera. 



The active longitudinal growth of the first frond and of the 

 first root of the young fern produces a constantly increas- 

 ing expansion of the surrounding tissue of the prothallium, 

 until the latter is ultimately unable to keep pace with the 

 increase in size of the young plant. The layer of tissue 

 surrounding the latter underneath, is ruptured transversely, 

 usually somewhat in front of the neck of the impregnated 

 archegonium. The frond immediately curves upwards, and 

 appears between the two flaps of the prothallium. Before 

 this period it has formed the rudiments of its lamina, which 

 in all ferns are much less divided in the young, than in the 

 full-grown plant. The first fronds of Potypodium valgare, 

 for instance, are not unfrequently undivided and lancet- 

 shaped ; more often however they are divided at the apex 

 into two portions of very unequal size. Contemporaneously 

 with the appearance of the first frond, the first root also 

 pierces downwards through the tissue of the prothallium. 

 Immediately after it makes its appearance it turns down- 

 wards into the ground.* 



If the second frond of the germ-plant is developed 

 very soon after the first, the surrounding cellular tissue of the 

 prothallium in the neighbourhood of, or above the point of 

 egress of the first frond, is pushed outwards and forwards, and 

 is ultimately broken through. Before the frond makes its 

 appearance out of this covering, the latter resembles a coni- 

 cal wart protruding into the indentation of the fore edge of 

 the prothallium : it is the body which Wigand (' Bot. Zeit.' 

 1849, p. 121) has described as the prolongation of the 

 midrib of the prothallium. 



* Yon Mercklin asserts (' Beobacht. am Prothallium der Parrnkr.' Peters- 

 burg, 1850) that soou after the appearance of the embryo in the interior of the 

 prothallium, a dark stripe becomes visible, passing from the base into the mass 

 of the prothallium, and expanding itself there. It contains a bundle of shortly- 

 jointed, striped vessels, the pointed ends of which reach to the neighbourhood 

 of the archegonia. The older the prothallium the more numerous are these 

 vessels, which, in their configuration, answer exactly to those of the large 

 vascular bundles of the first frond, and appear never to be wanting. I 

 find the prothallia of all the ferns which I have examined to be always com- 

 posed of homogeneous parenchyma, and to be devoid of vessels. I have not 

 the least notion what Von Mercklin's supposed striped vessels can be. 



