O THE FERNS AND 



The Stipes varies in different species chiefly in its relative length 

 and thickness, and in the presence or absence of scales and other 

 appendages. 



But it is in the fronds that the most conspicuous distinctions are 

 to be found, in their size, form and texture. They vary from not 

 more than about half-an-inch in length (as in Hymenophyllwm 

 cJieesemanii) to as much as 20 feet in the giant Mamuka {Cyathea 

 meduUaris). Some are quite simple in form, while others are cut 

 into innumerable little segments. In many HymenopJiyUums 

 (filmy-ferns) the texture is more delicate than tissue-paper, and 

 the tissues are formed of a single series or layer of cells; while 

 in Folypodium serpens the consistence is that of leather. The 

 surface is also very variable ; being glossy and quite destitute 

 of hairs {glabrous') in many species, or in others variously clothed 

 with hairs or scales of extremely different lengths, coarseness and 

 abundance. 



Among the most characteristic features in ferns are the shape and 

 position on the frond of the sorus or collection of capsules, and the 

 presence or absence of its involucre or covering membrane. We 

 have already seen that they are sometimes round and placed on the 

 back of the frond, or linear and close to the margin, or are in a 

 continuous line round the margin. In the Polypodiwms they are 

 naked, having no covering of any kind, while in Lomaria and 

 Asplenium they are seen to be furnished with some sort of an 

 involucre or indusium. The structure of this covering is very 

 various, but we can only consider it as it exists in a few genera. 

 (For its many modifications, the reader must be referred to Chap. 

 III.) In some cases, as in the Aspidiums, or Shield-ferns, the 

 involucre is a small circular disc fastened by the centre of its under 

 surface to the middle of the sorus (Plate III. fig. Sas), and it is from 

 this shield-like mode of attachment that both the scientific and 

 common names of these ferns are derived. In the allied genus 

 Nephrodium the involucre is kidney-shaped, but otherwise similar. 

 In these, and in Asplenium, the involucre is a specially-developed 

 structure of the epidermis of the frond. In Hymenophyllum and its 

 allied genera it appears to be an outgrowth of the tissue of the 

 frond itself. In them the veins are produced beyond the margins of 

 the frond, and the capsules are clustered near their extremity, and 



