Dr. G. Ogilvie on the Forms and Structure of Fern-stems. 403 



ther. By tins disappearance of the common connecting portion 

 of the rhizome, the later branches assume the position of inde- 

 pendent plants, and the forest of fronds covering the tnrf may 

 be compared to the vigorously vegetating leaf-shoots of a sub- 

 terranean shrub whose trunks have already mouldered away. 



Roots of this description occur in all our Polypodies (PI. VIII. 

 figs. 1,2), in the Bracken, in Hymenophyllum, and probably also 

 in Adiantum, Trichomanes, and in Lastrea Thelypteris. 



The common Polypody, however, has one peculiarity, in which 

 it stands alone among our indigenous Ferns, — that its petioles 

 break off by articulations, so that the older portions of the stem 

 do not bear the stumps of former leaf-stalks, but only cicatrices 

 marking their insertions. 



To this variety the form described as the simple caudex stands 

 in the most marked contrast. Its characteristic features are the 

 number and spiral arrangement of the petioles of the leaves. At 

 its growing extremity these form one of the graceful circles of 

 bright green foliage, which are so pleasing a feature in the vege- 

 tation of our larger Ferns. The axis itself is sometimes of con- 

 siderable thickness, but its real dimensions are not distinguish- 

 able at first sight, from its being so completely ensheathed in the 

 persistent bases of the decayed fronds of former years. Its direc- 

 tion of growth varies from horizontal to vertical (PI. X. figs. 7, 8). 

 In the former case, it creeps along or just under the surface of the 

 ground, and forms a more or less considerable angle with the 

 terminal crown of fronds, the axis of which is always vertical, 

 owing to a corresponding curvature of the petioles at their points 

 of origin. In cases, again, where the axis of the stem approaches 

 the vertical, as in some foreign species of Blechnum, Struthio- 

 pteris, &c, it gradually rises above the ground in the course 

 of growth, and assumes on a small scale the features of the 

 arborescent form, the terminal circle of leaves corresponding 

 with that elegant palm-like crown of foliage which is so promi- 

 nent a characteristic of the Tree-ferns. The correspondence 

 between such a caudex and the corm of a Tree-fern is brought 

 out more clearly by cutting off all the bases of the old fronds at 

 their points of insertion, when we obtain a nearly cylindrical 

 stem, marked with rhomboidal scars, answering to those which, 

 in most arborescent species, indicate the points of insertion of 

 fallen petioles (PI. X. fig. 9). 



In this operation we have to clear away numerous radical 

 fibres, as well as the petioles of old fronds, for the rootlets are 

 equally abundant in this as in the former or stoloniferous 

 variety, though, from the closeness of the leaf-stalks, between 

 whose insertions they originate, they form but a secondary fea- 

 ture, except at the lower pails of the corm, where the decay of 



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