202 



THE MARATTIALES 



teris is really built up of leaf traces, the so-called "compensating segment" being 

 nothing more than the lower part of a leaf trace which higher up emerges as a meshed 

 segment from the outer zone and passes into the base of the leaf. 



The structure of the petiole (fig. 182, //) is like that of Marattia, except that 

 the vascular bundles are more numerous and are arranged in several circles, inclos- 

 ing one or two small bundles in the center. A cross-section of a leaflet (fig. 182, B) 

 shows the vascular bundle which traverses the midrib to be horseshoe-shape in 

 section, having a central mass of tracheids, with reticulate or scalariform markings, 

 and surrounded by the phloem, largely made up of large sieve tubes, but having 

 also protophloem cells and bast. The ground tissue is composed largely of paren- 

 chyma, but on both sides below the epidermis is a conspicuous band of collenchyma. 

 In the larger divisions of the leaf the collenchyma, as in Marattia, is replaced by 

 sclerenchyma. The structure of the lamina is very much like that of Dancea, but 

 the palisade parenchyma is even better developed. 



A comparison was made of the leaves of two forms (species T) of Angiopteris, 

 one from Australia, the other from Ceylon. These showed several notable differ- 

 ences. The leaflets of the Ceylonese specimens were thinner and sharply serrate, 



Si-P 



Fig. 182. — Angiopteris. 



A. Section of petiole from an adult sporophyte, somewhat reduced. 



B. Section of a leaflet, col, collenchyma; p, palisade tissue. X14. 



C. Part of sporophyll, showing the sori. X4. 



while in the Australian specimens the serrations were almost wanting, except at the 

 tapering apex of the leaflet. There were also marked anatomical differences. In 

 the Australian form the paHsade cells are very much elongated and are separated 

 from the epidermis by a layer of colorless hypodermal cells, and the spongy meso- 

 phyll of the lower part of the leaf is decidedly more compact than in the form from 

 Ceylon. In the latter, the palisade cells are noticeably shorter and abut directly 

 upon the epidermis. Undoubtedly the differences in the anatomy of the two forms 

 are associated with the difference between the moist, tropical climate of Ceylon and 

 the drier and cooler climate of Australia, and perhaps do not necessarily imply that 

 the two species are distinct, although it is highly probable that such is the case. 



The roots originate, for the most part, in the inner zones, but a few may arise 

 m connection with the bundles of the outer zones and must necessarily traverse a 

 very large amount of tissue before they finally emerge. 



Russow (Russow 1) described two sorts of roots— earth roots which were 

 branched and had but five xylem rays completely lignified to the center of the bun- 

 dles, and aerial roots which were much larger and unbranched and had twelve to 



