INTRODUCTION 23 



strand, the mostly highly developed element of the 

 latter being the sieve-tubes found in Macrocystis 

 (Laminariacecv). The sculpturing of outward form 

 reaches its highest point in the differentiation (1) of 

 a root-like holdfast, which, however, is not an organ 

 of absorption unless possibly in the case of certain 

 partially parasitic forms (cf. Notheia, Choreocolax, 

 Figs. 5rf, 64) ; (2) of a stem and (3) of leaf-like appen- 

 dages. From this type there are varying intermediate 

 forms down to the wholly undifferentiated type, which 

 occurs among both multicellular and unicellular 

 forms. These intermediate forms may be placed 

 into two categories those exhibiting a root-like 

 differentiation from an otherwise unspecialised body, 

 and those in which there is merely a distinction 

 between base and apex. In some of the lower 

 multicellular Algae all the cells are alike, and equally 

 capable of vegetative and reproductive functions. 

 Among the unicellular forms there are those which 

 exist free singly, and others united into a kind of 

 spurious tissue or colony by a common investing 

 mucilaginous cell-wall, and occurring either in rows 

 or filaments, or in more or less indefinite masses. 

 The highest development attained by the unicellular 

 forms, if they may be so termed in this connection, 

 is to be found in the multinucleate group of 

 Siphonece, which includes many forms with differen- 

 tiated root-like appendages and leaf-like shoots, and 

 others in which the specialisation is carried so far 

 as to represent leaf-like, stem-like, and root-like 

 organs. 



Growth in length is either (1) apical, and effected 



