BY \V. A. HASWELL, M.A., D.SC. 



1027 



ai-e elongated in the direction of the long axis of the stem ; the 

 outer wall, in the leaf-bearing ])art of the stem, but not in the 

 basal part, is irregularly thickened, leaving rounded or slit-like 

 depressions. The cortical tissue is strongly sclerenchymatous in 

 its outer part, with abundant cell-contents, and with numerous 

 intercellular spaces. Internally the walls of the cells are thinner 

 and the cell contents scanty, so that this part of the cortex may 

 be regarded as representing the endodermis. Surrounding the 

 central vascular bundle is a ring of a dark brown homogeneous 

 substance, which is so arranged that it forms continuous branching 

 and anastomosing longitudinal lines breaking through apparently 

 from cell to cell, and thus constitutes a network enclosing the 

 Inindle. This brown layer is present in nearly all parts of the 

 stems both creeping and aerial ; it varies in thickness and may 

 sometimes be found to be entirely absent for a short space. When 

 at its thickest it occupies about four layers of cells ; near the 

 growing point of the underground stem it sometimes breaks 

 through into the interior of the vascular bundle, and fills the 

 interior of some of the spiral vessels. This brown matter is solid, 

 and is quite insoluble in water, cold or boiling, in boiling absolute 

 alcohol, ether, chloroform, turpentine, and liquor potassise ; a 

 similar substance occurs in Psilotum, as will be noticed below. 

 Internal to the brown layer there is nearly always a single layer 

 of thin-walled cells not differing from the other endoderm cells, but 

 mai'ked off from those immediately external to them by not con- 

 taining any of the brown mattei". There is a single, central, 

 cylindrical vascular bundle. In the basal leafless part of the 

 aei'ial stem the scalariform tracheides usually form in transverse 

 section an irregular incomplete ring or a series of groups circularly 

 arranged, surrounded by and enclosing elongated elements with 

 thin cellulose walls, some with long narrow nuclei, others without 

 nuclei, apparently sieve-tubes. The central selerenchyma found in 

 Psilotum is absent. In the leaf-bearing part of the stem the vessels 

 occupy a centi'al position surrounded by the phloem elements. 

 The vessels are smaller than in Lycopodium, the largest being 

 little over j^q of an inch in diameter. In transverse sections of 



