XI] 



VENTILATION 



203 



Pneumatophores 



The frequent sclerotic strengthening of the surface-tissues of the stem and 

 leaf-stalk in Ferns has led to the establishment o^ ventilating areas, providing 

 for the necessary gaseous interchange which the dense sclerotic bands would 

 otherwise prevent. These areas are often visible as pale lines or patches. 

 A familiar example of the former is seen in the Bracken, where lateral lines 

 corresponding in position to the margins of the dorsiventral organ are trace- 

 able down the petiole, and onwards upon the rhizome. Sections show that 

 here the sclerenchyma is replaced by highly ventilated parenchyma, covered 



Fig. 192. Hairs from base of the leaf 

 of Gleichenia pectinata. Each is 

 seated on a massive emergence, 

 from which it is easily detached, 

 the emergence remaining as in 

 Cyatheaceae. ( x 50.) 



Fig- 1 93. External luitacc uf nIciii of Alsophila crinita, 

 Hk., cleared of scales, showing two leaf-scars, and 

 the large lenticel-like pneumathodes, or pneumato- 

 phores, which traverse the outer sclerenchyma. Re- 

 duced. 



by epidermis bearing stomata. Similar ventilating areas are seen in the large 

 petioles of Alsophila, Saccoloma, and Thyrsopteris (Figs. 161, 162). They 

 correspond in position to the deep lateral involutions of the vascular tracts. 

 In the large Tree Ferns a somewhat similar perforation of the sclerotic rind 

 appears at the base of the leaves, and on the surface of the stem itself. 

 Round or oval areas irregularly arranged are seen in the adult state filled 

 with pulverulent tissue (Fig. 193). In the young condition the tissue was 

 lacunar parenchyma, covered by epidermis with stomata. These areas serve 

 for ventilation through the sclerotic covering, in much the same way as do 

 the lenticels of Flowering Plants. 



