LAMINARIA. 63 



2. A massive zone which constitutes the chief bulk of the stipe : 

 in the centre of it may be recognised 



3. A central medullary patch of narrow-oval form, which 

 indicates the originally flattened character of the stipe itself. 



From material previously hardened in alcohol, cut transverse 

 sections of the stipe, so as to include all the tissues to the centre : 

 mount in glycerine, and examine under a low power : note 



1. The superficial tissues of the external cortex, which may 

 be more or less brown according to age, both in the walls and 

 the cell-contents. In old specimens there will be found large 

 mucilage-cavities at the inner limit of this tissue, which are 

 apparently of schizogenetic origin. 



2. A meristematic zone, or cambium, in which the cells 

 correspond in appearance, and in their arrangement in radial 

 rows, to those of the cambium of vascular plants : by the activity 

 of this layer the stipe undergoes a secondary thickening, 



3. Passing inwards, a bulky mass of ground tissue is found 

 consisting of cells arranged with considerable regularity in radial 

 rows, pointing to an origin from the cambium : the cell-contents 

 are for the most part inconspicuous, and the walls of somewhat 

 irregular thickness. 



4. Centrally is the oval area or medullary tissue, above noted 

 with the naked eye : the tissue here is traversed by numerous 

 hyphal threads, similar in nature to those in.Fucus. 



Cut longitudinal sections of the stipe, and, treating as before, 

 observe 



1. That the external cortex, cambium, and ground tissue con- 

 sist of cells which are of no great length, with rounded or square 

 ends, and walls with a few pits. 



2. That the mucilage-cavities do not extend far in a longitudinal 

 direction. 



3. That the medullary tissue consists chiefly of hyphal threads 

 closely interwoven, with thick lateral walls and thin transverse 

 septa. Note especially at the periphery of this tissue how the 

 hyphal threads originate as papillae from the cells of the ground 

 tissue, and then push their way as they grow between the 

 neighbouring cells. 



Cut transverse and longitudinal sections of one of the segments 

 of the frond, being careful to avoid the reproductive patches. 



