FERN STRUCTURE. 



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

 form the mouth of the passage and expand when moist, allow- 

 ing the atmospheric gases and watery vapor to escape or enter 

 but close the entrance by contraction in time of drought. The 

 stomata are not confined to the fronds, but are found to a 

 greater or less extent on all aerial portions of ferns and higher 

 plants, as well as on subterranean stems. 



78. Asexual Reproduction. Besides the ordinary meth- 

 ods of sexual reproduction discussed in Chapter IV., most ferns 

 are propagated by growth of the rootstock under ground, giving 

 rise to a succession of fronds each season. In addition to this, 

 which is common to all perennial plants, there are some meth- 

 ods of reproduction that deserve attention. The first is by 



79. 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 un- 

 common to see a large number in vari- 



ous stages of growth rising from a sin- 

 gle frond. This peculiarity is common 

 among several species of Asplemum, 

 especially A. furcatum Thunb., and 

 will be sometimes 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 devel- 

 opment of the axillary buds of the tiger- 

 lily. 



SO. Another method is seen in the 

 walking-leaf (Camptosorus rhizophyllus), 

 in which the long, attenuated, simple 

 fronds bend over and take root in the 

 adjoining soil in a manner quite analo- FlG ^.Camptosorus rhi- 



f zophyllus Link., reduced, 



gous to the propagation of strawberries showing peculiar method of 



by runners (Fig. 18). The same method 



of rooting at the apex has also been noticed in Asplenium 



pinnatifidum, A. ebeneum, and Phegopteris reptans. 



