124 THE ALGAE 



(i) Simple or hmmsitQ parenchymatous thallus (e.g. Punctarid). 



(j) Improved parenchymatous structure with internal differentia- 

 tion of the tissues (e.g. Laminariales). 



Many of the simpler types, whether reduced or not, exhibit the 

 condition of heterotrichy similar to that found in the Chaeto- 

 phorales, but this is a feature that will be discussed elsewhere 

 (cf. p. 315). The thalli may also reach a relatively large size and 

 under these circumstances additional support is obtained as 

 follows : 



(i) Increase in wall thickness (Stypocaulon) or the production of 

 a firmer cellulose material {Sphacelaria). 



(2) Twisting and roUing of the threads together. 



(3) Development of root branches or haptera. 



(4) The appearance of descending and ascending corticating 

 filaments. 



(5) Multiseptation takes place in a longitudinal direction. 



(6) Development of internal hjrphae. 



The characteristic method of growth in the group is by means of 

 an intercalary meristem at the base of a hair. This is termed tri- 

 chothaUic growth. In some of the more advanced orders tricho- 

 thallic growth has been replaced by an apical cell or an apical or 

 marginal group of cells. Branching may proceed from any cell, and 

 it frequently takes the form of a regular or irregxilar dichotomy, 

 though in the Fucales a spiral arrangement may be found. The 

 cells vary greatly in size but they always have distinct walls, which 

 are usually composed of cellulose on the inside and pectin outside, 

 and although they are commonly uninucleate occasionally they 

 become multinucleate. Plastids are also present and when the 

 brown pigment has been removed by boihng, the thallus then be- 

 comes green. Usually each cell contains several plastids, which are 

 usually parietal and discoid though there is some diversity of 

 form. Although pyrenoids have been described for some species 

 they do not appear to be wholly comparable with those in other 

 algae. The pigments are not the same as those in the higher 

 plants because chlorophyll b is replaced by chlorophyll c and there 

 are a number of accessory xanthophylls (see p. 2). The products 

 of assimilation are sugar, alcohols, fats and complex polysaccha- 

 rides but not starch. A characteristic feature of the Phaeophyceae 

 is the presence of colourless, highly refractive vesicles known as 

 fucosan vesicles or physodes. These contain a material known as 



