26 OUR NATIVE FERNS AND THEIR ALLIES. 



74-. Frond. — In the HymenophyllacE/E the frond con- 

 sists of a single layer of cells. This condition is also found in 

 the leaves developed along the axis of growth among the 

 mosses to which this sub-order is related in some of its forms. 

 In all other rerns there are several layers of cells variously 

 compacted together, and forming all the varieties of texture 

 — membranous, herbaceous, coriaceous and fleshy. The epi- 

 dermis is usually easily separable from the underlying tissue, 

 when its peculiar markings can be studied. 



75. From the epidermis a great variety of appendages are 

 developed which are all modifications of hairs, and are all in- 

 cluded under the term trzckomes, however different in appear- 

 ance or distinct in function. These are not confined to the 

 frond, but develop here their greatest variation. They are fre- 

 quently found on the roots, the rootstock, and the stipe, under 

 the form of root-hairs or scales of various forms. Scales are 

 especially abundant in certain forms of Dryopieris, as well as in 

 Polystichum, Cheilanthes, and other genera. 



76. Trlchomes. — On the fronds the trichomes may be de- 

 veloped as simple unarticulated or articulated hairs, consisting 

 of one or two cells at most. They may appear as stalked 

 glands like those that arise from the stipe of Cheilaiithes 

 CoopercE or the margin of the indusium of Dryopteris spijmlosa, 

 var. intermedia ; or they may be developed into scales of in- 

 tricate cellular structure like those on the under surface of cer- 

 tain forms of Cheilanthes, particularly C. Fendleri and C. Cleve- 

 landii. In the true ferns the sporangia are specialized, tri- 

 chomes developed in clusters (sori) along the veins, or spread 

 over the entire surface of the frond, or even arranged in spikes 

 or panicles. The epidermis also develops an excrescence 

 known as the indusium, which consists of a single layer of cells, 

 and is variously arranged as indicated in Chapter III. In some 

 cases a false indusium is provided, which is not a growth from 

 the epidermis, and may consist of several layers of cells. 



77. Stomata. — If the epidermis covering the under surface 

 of a fern be examined under a high magnifying power, peculiar 

 structures will be seen in the form of semi-elliptical or crescent- 

 shaped cells connected at their apices and separated between. 

 These are the guard-cells of stomata which control the open- 



