X THE HOMOSPOROUS LEFTOSPORANGIATAl 381 



distinguish any well-marked sieve-tubes, but it was mainly com- 

 posed of bast fibres and cambiform cells, and in llcniipJilcbinui 

 {Trichomancs) Hookcri the phloem is absent from the very 

 much reduced smaller veins. This is possibly an intermediate 

 condition between the normally develcjpcd bundles of the veins 

 of most species and the scncalled pseudo-veins, in which there 

 is no tracheary tissue developed, but which in tbeir r)rii^in cor- 

 respond to the ordinary veins. The petiole always has a single 

 vascular bundle, usually of typical concentric structure, but in 

 the section Hcmiphlchmm Prantl states that it is collateral. 

 The ground tissue of the petiole is largely composed of scleren- 

 chyma like that of the stem. 



The Roots 



The development of the roots has been studied only in a 

 very few species. Bower (11) states that in T. 7'adicans and H. 

 demissmn it ''conforms to the normal type for the root of lep- 

 tosporangiate Ferns, as described by Nageli and Leitgeb," but 

 does not go into details, and Prantl makes an equally brief 

 statement. While lateral roots are completely wanting in the 

 section Hcmiphlebmni, where their place is taken by leafless 

 branches, in most of the other forms they are developed in 

 considerable numbers. There is, according to Prantl, great 

 variation in the arrangement of the parts in the vascular cyl- 

 inder. Thus while all the species of HyinenopJiylliiin have 

 diarch bundles, that of Trichomanes pyxidifcrum is monarch, 

 while in one species, T. brachypiis, as many as nine primary 

 xylem masses are found. The Marattiacese alone, among the 

 other Ferns, show such great variability. 



Trichomes occur, but not so abundantly as in most of the 

 Leptosporangiatse. They have usually the form of hairs, 

 which are either temporary (those formed on the margins of 

 the young leaves) or persistent for a longer time, like those 

 that cover the end of the stem apex and bases of the petioles in 

 many species. 



The Sporanghim 



All of the Hymenophyllace?e agree closely in the position of 

 the sporangia, whose development has, howe\er, been studied 

 in detail only in Trichomanes ; but from the close correspond- 



