53 ELEMENTARY BIOLOGY. [CHAP. 



walled, elongated, parallel-sided cells, internal to which lie 

 elongated tubes devoid of protoplasm, and frequently con- 

 taining air. In the majority of these tubes, and in all 

 the widest, the walls are greatly thickened, the thickening 

 having taken place along equidistant transverse lines. The 

 tubes have become flattened against one another, by mutual 

 pressure, so that they are five- or six-sided ; and, as the 

 markings of their flattened walls simulate the rounds of a 

 ladder, they have been termed scalariform ducts or vessels. 

 The cavities of these scalariform ducts are divided at 

 intervals, in correspondence with the lengths of the cells 

 of which they are made up, by oblique, often perforated, 

 partitions. Among the smaller vessels, a few will be found, 

 in which the thickening forms a closely wound spiral. 

 These are spiral vessels. 



The rachis of a frond, so far as it projects above the 

 surface of the ground, is of a bright green colour ; and, in 

 transverse section, it presents a green ground-substance, 

 interrupted by irregular paler markings, which are the trans- 

 verse sections of longitudinal bands of a similar colour. 

 There are no brown spots or bands. Examined micro- 

 scopically, the ground-substance is found to be composed 

 of polygonal cells containing chlorophyll. These are 

 invested superficially by an epidermis, composed of elon- 

 gated cells, with walls thickened in such a manner as to 

 leave thin circular spots here and there. Hence, those 

 walls of the cells, which are at right angles to the axis of 

 vision, appear dotted with clear spots; while, in those 

 walls of which transverse sections are visible, the dots are 

 seen to be funnel-shaped depressions. 



The pale bands are vascular bundles containing scalari- 

 form and spiral vessels. The outer layer investing each 

 is chiefly formed of long hollow fibres with very thick 



