Structure. 43 



develops an excrescence known as th^ 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. 



In the OphioglossacejE the sporangia are not developed from 

 the epidermal cells, but arise from a transformation of the interior 

 tissue of the leaf. This with other characters as clearly defined 

 serves to separate these anomalous plants from the order Filices 

 to which they were formerly appended. 



81. 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 stomata or breathing pores, and are merely open- 

 ings to the air chambers of the plant. The two elliptical cells form 

 the mouth of the passage and expand when moist, allowing the 

 atmospheric gases and watery vapor to escape or enter, but close 

 the entrance by contraction in tinie of drought. The stomata are 

 not confined to the fronds but are found to a greater or less ex. 

 tent on all aerial portions of ferns and higher plants as well as on 

 subterranean stems. In many of our species of Cheilanihes and 

 Notholtsna there is an additional provision against drought in the 

 copious tomentum or chaff which is borne on the under surface of 

 the fronds. 



82. Buds and Bulblets. — In a few species of conservatory ferns 

 adventitious buds are produced on the surfaces of the fronds. 

 These soon develop into young ferns and it is not uncommon to 

 see a large number in various stages of growth rising from a single 

 frond. This pecuHarity is common among several species of As- 

 plenium, especially A. furcatum, Thunb., and will sometimes be 

 found to occur among some of our native species. Bulblets are 

 found in the axils of the upper pinnae of Cystopteris bulbifera 

 which often fall to the ground and develop into new plants after 

 a manner analogous to the development of the axillary buds of 

 the tiger lily. 



83. Vernation. — As has been before stated, ferns are rolled 

 in the bud from the apex downward (circinate), distinguishing 

 them from the higher forms of vegetation. Among the Ophio- 

 glossacEjE, however, the vernation is either straight, inclined at the 

 apex of one or both segments, or else the fertile segments are 

 folded on the main stalk, making the vernation wholly inclined. 



