INTRODUCTION 23 
strand, the mostly highly developed element of the 
latter being the sieve-tubes found in Maecrocystis 
(Laminariacee). 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 (¢f Notheia, Choreocolas, 
Figs. 5d, 64) ; (2) of a stem and (8) 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 Algze 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 
Stphonec, 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 
