AUSTRALIAN FEENS. 43 



mate, but also from the degree of light or moisture to which they 

 are subjected. Persons who are in the habit of observing ferns 

 in a living state, are well aware of this peculiarity, and they 

 might in some instances collect individuals of the same species so 

 unlike each other, that the systematic botanist in Europe, who 

 judges simply from the specimens before him, might regard them 

 as distinct. Brown's Pterisfalcata, now Pell&afalcata, is a plant 

 of this varying character, and probably the cognate fern Adiantum 

 paradoxum, (Pellcea paradoxa of Hooker) is a mere variety of it, 

 for when we can find so many intermediate forms connecting the 

 two together, it seems almost impossible to avoid the conclusion 

 that they should be regarded as the same species. In the course 

 of our investigation, we shall find several parallel cases, so that it 

 may be well to bear in mind, that although " a species comprises 

 all the individual plants which resemble each other sufficiently to 

 make us conclude that they are all, or may have been all, des- 

 cended from a common parent," yet, " individuals may often 

 differ from each other in many striking particulars." 



The order of Filices or Ferns (which Sir W. Hooker justly ob 

 serves are more easily to be recognised than described) consists 

 of flowerless plants, furnished with fronds or leaves which bear on 

 some part of their surface, usually the lower or under one, the 

 spores or seeds by which they are propagated. Although ferns 

 have no flowers, they bear great abundance of seed- like bodies, 

 which are contained in sporangia or spore cases. These cases 

 are generally surrounded by an elastic band or ring, which, when 

 they have reached maturity, bursts by an irregular fissure, and 

 the seeds or spores, in the shape of fine dust, are scattered to the 

 winds in countless numbers. The separate masses of the seed 

 cases are termed sori, and that organ which in many genera 

 covers the sori in the earlier stages of growth, is called an ix- 

 diisium or involucre. As many ferns are destitute of this organ, 

 the presence or absence of an indusium affords one of the means 

 of dividing them into groups or sub-orders ; whilst the shape of 

 that organ, combined with its position as to the veins of the frond 

 suggests a natural method of subdividing some of the groups into 

 genera. This is a matter of difficulty, and Botanists will differ 

 in their system of classification, according to the relative impor- 

 tance which they attach to particular organs : some for instance, 



