GROUP I. THALLOPHYTA : ALGM. 



223 



of the body being merismatic (e.g. Spirogyra, Ulothrix, Ulva) ; 

 in other cases there is a definite intercalary growing- point, as in 

 Laminaria (Fig. 159) and other Phaeophycea?, in some of which 

 growth is tricJiothallic, that is the shoot terminates in a single 

 multicellular hair (e.g. Desmarestia, Fig. 160), or in several hairs 

 (e.g. Cutleria), or in a tuft of hairs (e.g. Carpomitra Cabrerce), and 

 towards the base of each such hair lie the merismatic cells which 

 constitute the inter- 

 calary growing- point. 

 In some few cases (e.g. 

 Volvocoidere, Botry- 

 dium) there is no 

 growth after the em- 

 bryo-stage, as the 

 whole protoplasm of 

 the body passes over 

 into the adult condi- 

 tion. 



The primary root is 

 never developed in due 

 proportion to the 

 shoot ; consequently, 

 in order to ensure the 

 attachment of the 

 plant, adventitious 

 roots are very com- 

 monly formed on the 

 shoot, and when the 

 shoot is dorsiventral 

 unicellular root -hairs 

 are commonly de- 

 veloped on the surface 

 in contact with the 

 substratum. In some 

 cases special organs of 

 attachment (haptera, 

 see p. 66) are developed on the shoot ; they may be adhesive discs 

 borne on the ends of branches of the shoot (e.g. Plocamium coccin- 

 eum], or root-like out-growths as in Laminaria bulbosa, where at 

 the base of the shoot, a large umbrella- shaped out- growth is formed, 

 bearing numerous haptera on its upper and outer surface. 



FIG. 159. Laminaria dijitata (after Harvey; much 

 reduced;. The intercalary growing-point is situated 

 at the junction of the old frond (b) with the new frond 

 (a) ; a is gradually splitting into segments. 



