204 PRACTICAL BOTANY 



2. Immediately below the limiting layer, and separated from 

 it by an irregular line, is a parenchymatous tissue, consisting of 

 cells which appear nearly square in the transverse section ; 

 each cell has plentiful protoplasm, a nucleus, and several chloro- 

 phyll corpuscles : the walls are more or less thickened, swollen, 

 and stratified : here and there are to be seen pits closed by a 

 thin, highly-refractive pit-membrane. These two tissues, (i) and 

 (2), together constitute the cortical band above recognized 

 under a low power. 



3. The above tissue graduates off without definite limit into 

 the medulla, of which the chief characteristic is the excessive 

 bulk of the swollen cell-walls : here and there " trabecular " 

 filaments will be found running in the plane of section. If 

 sections be stained with Schulze's solution the firmer cell-wall 

 stains pale blue, but the swollen matrix does not stain. 



Occasionally a section may be found which has passed 

 through one or more of the sterile or neutral conceptacles. If 

 the conceptacle was young, it would be found to be a still 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, and the hairs much longer. 



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 trans- 

 verse septa : this tissue merges gradually into 



3. The medulla, which consists of " trabecular " tissue : 

 here longitudinal rows of cells constitute filaments resembling 

 the hyphas of Fungi, with numerous thin transverse septa, to 

 which the protoplasmic contents closely adhere. 



IV. In order to study the process of thickening of the midrib, 

 cut transverse sections successively of older (i.e. lower) parts of 



