FUCUS SERRATUS 397 



closed cavity of considerable size, filled with mucilage, 

 and with hairs, which originate from single cells of the 

 tissue lining the cavity ; in sections of older parts the 

 ostiole would be found widely open, the hairs much 

 longer, and protruding out of the conceptacle : these 

 hairs may be observed even with the naked eye on the 

 older parts of the thallus. 



III. Cut longitudinal sections of a young part of a 

 thallus (i.e. close to the apex of a branch), mount in 

 pure glycerine, and examine under a high power : 

 recognize, as in the transverse sections, the cortical 

 band, consisting of 



1. The limiting layer, which presents similar 

 characters to those seen in transverse sections. 



2. The inner parenchyma : note that the cells of 

 the inner part of this tissue are arranged in longitudinal 

 rows, having relatively thick, occasionally pitted, lateral 

 walls, and thin transverse septa : this tissue merges 

 gradually into 



3. The medulla, which consists of " trabecular " 

 tissue : here longitudinal rows of cells constitute fila- 

 ments resembling the hyphse of Fungi, with numerous 

 thin transverse septa, to which the protoplasmic contents 

 closely adhere : as already seen in transverse section, the 

 longitudinal walls consist of a firm, highly refractive 

 cellulose coat, embedded in a mucilaginous matrix. 

 Note the frequent lateral communications of the fila- 

 ments, and compare them with the pits in the cortical 

 tissue, to which they undoubtedly correspond. 



IV. In order to study the process of thickening of 

 the midrib, cut transverse sections successively of older 

 (i.e. lower) parts of the stalk : treat as before, and 



