3 8o 



BOTANY OF THE LIVING PLANT 



Laminaria, where a new frond is formed each season between the 

 old one, which is thrown off, and the persistent stalk (Fig. 281). 



In simple forms the cells are all alike. Soft cell-walls surround a 

 uni-nucleate protoplast, which includes simple brown chromatophores. 

 The plastid pigments are chlorophyll, xanthophyll and carotin, to- 

 gether with fucoxanthin which masks 

 the others and is peculiar to the group. 

 The products of photosynthesis are 

 soluble carbohydrates like laminarin 

 or mannitol. In larger forms the 

 tissues are differentiated. For in- 

 stance, in Fucus the cells of the super- 

 ficial layer are thin-walled, and divide 

 actively ; they are covered externally 

 by a layer corresponding to cuticle. 

 They are the chief seat of photosyn- 

 thesis, and of tissue-formation. Pass- 

 ing inwards from this layer the 

 mucilaginous cell-walls become more 

 and more swollen, so that the deeper- 

 seated tissues of an old thallus con- 

 sist of a bulky mucous matrix, in 

 which the cells themselves appear as 

 a complicated network (Fig. 282). 

 Centrally there is a firmer conducting 

 cord, which is well defined in old 

 stalks of the larger Tangles. It 

 contains manv tubes with sieve- 

 structure and callus, closely com- 

 parable to the sieve-tubes of Vascular 

 Plants, and serving like them for 

 transport. In large stalks an ill-defined cambial activity provides 

 for thickening and increased mechanical strength. This is still 

 further secured by " intrusive hyphae," which burrow through the 

 softer tissues, and brace them together. In this way they acquire 

 their tough and resistant but yet pliant character. We thus see 

 that both in external form and internal structure the Brown Sea- 

 weeds cover a wide range, from the simple to the complex. 



In their sexual propagation they also show an advance, which runs 

 in some degree parallel with their structural progress. The propagative 

 cells are produced in sporangia and gametangia. The former which 



Fig. 281. 



Laminaria, thallus showing a new frond, 

 intercalated between the stalk and the old 

 frond, which is being thrown off. (Re- 

 duced. ) (After Strasburger.) 



