BRITISH SEA-WEEDS. 



ranged lengthwise, either packed closely together or se- 

 parated by intervening layers of gelatine, and is spoken 

 of as the axis of the frond. The cells of the peri- 

 pheric series are for the most part smaller, and are dis- 

 posed horizontally. Both these series are subject to 

 numerous modifications in different genera, and in many 

 instances subsidiary series are interposed. 



The gelatine, which enters largely into the composi- 

 tion of the fronds of Sea-weeds, varies very much, both 

 in quantity and quality, in different genera. In some it 

 is almost as limpid as water, and in others nearly as 

 firm as cartilage. In some, as for instance Dudresnaia, 

 it so overpowers the other constituents of the frond that 

 the whole plant seems to be but a mass of slime, while 

 in contrast to this there are many genera where it is 

 scarcely perceptible. 



It is this gelatine which gives substance to the frond, 

 and hence those plants, where the quantity is small, are 

 said to be membranaceous ; those where it is abundant 

 and fluid, gelatinous ; and those where it is firm, carti- 

 laginous. The shrinking which all Sea- weeds undergo 

 in the process of drying is due to the expulsion of this 

 element of their composition. 



The mode of growth is not the same in all Sea- weeds. 

 The fronds of some genera grow only at the tip, and 

 their youngest shoots are, therefore, those which are 

 furthest from the root. In the Laminaria, on the other 

 hand, the new growth takes place at the base of the old 

 frond, and for this reason it is almost impossible to find 

 any but very young specimens with perfect tips. In 

 several species of this genus a new frond is formed, and 

 the old one thrown off, every season; and specimens of 

 L. digitata, showing this process in progress, are not 



