INTRODUCTION. 



scope, it will be found to consist of four distinct portions 

 concentrically arranged, which Lamouroux, who first ob- 

 served minutely the anatomy of these plants, compares, 

 perhaps too fancifully, to the epidermis, bark, wood, and 

 medullary sheath of exogenous plants. The central por- 

 tion, corresponding to the medulla, occupies fully a third 

 of the diameter of the branch, and is composed of densely 

 packed, longitudinal, parallel fibres, or strings of cells, 

 firmly cohering into one compact mass. Outside this is a 

 much less dense layer, of a paler colour, composed of 

 branched, anastomosing fibres, partly horizontal and partly 

 vertical, inextricably laced together ; and surrounding 

 these, which represent the wood, is a third and much 

 denser and darker coloured layer (bark), which is altogether 

 composed of horizontal, radiating, simple fibres, very 

 densely packed together. Outside this portion, and form- 

 ing the outer coat of the frond, is a very thin layer of cells, 

 which is frequently but loosely attached, and separates 

 much in the manner of an epidermis. Something similar 

 to this, which we may call the analogy of the Exogenous 

 type among Algae, is the structure of many of the larger 

 kinds, both of the red and olive series, but minor variations 

 occur in the comparative substance of the different layers. 

 Thus, in some the centre is very loose and gelatinous, with 

 merely a few scattered fibres, while the outer coat is very 

 dense. The second circle (that representing wood) is ne- 

 ver, I believe, so dense as the others, and very generally 

 consists of branched, interlacing and colourless fibres, and 

 from it fructification generally, if not always, proceeds. 

 Another common form of the cell is that of an irregular, 

 very rarely regular, polygonal solid, resulting from the 

 lateral and vertical pressure of a mass of spherical cells. 

 This form is found generally in the Ulvce, and in most spe- 

 cies having large expanded leaves, especially among the 



