36 MANUAL OF BOTANY 



special discs or outgi'owths from the stem known as hapfera. 

 The shoot in many cases may show differentiation into stem 

 and leaf, sometimes of very delicate form. Instances may be 

 found in Caulerpa, Char a, Sargassum, and others. 



The mode of growth in length of the thallus or thalloid 

 shoot also varies a good deal. There are often definite growing 

 points, which may be either apical or intercalary. When they 

 are apical they sometimes consist of a simple apical cell, as 

 they do in the Ferns and their allies ; or they may be composed 

 of a nmnber of cells. These may be disposed in a series along 

 the margin of the thallus, or may form a group resembling the 

 apical meristem of the Phanerogams. In the ccenocytes, though 

 these grow at their apices, no apical cell is differentiated. When 

 the growth in length is intercalary there is often a definite 

 growing zone in the frond, but sometimes this is not the case, 

 any cell being capable of division. Sometimes the growing point 

 consists of a terminal hair, or collection of hairs, the basal cells 

 of which are merismatic. In most filamentous forms, any cell 

 of the filament can divide, and so increase the length of the 

 filament. 



Where secondary growth in thickness takes place it may be 

 brought about in two ways. In the stalks of some of the 

 Laminarias a secondary meristem or cambium is developed, 

 either in the epidermal cells or those immediately beneath them. 

 This behaves as in woody Dicotyledons, producing new tissue on 

 both sides. The external tissue forms a pseudo-bark or rind, 

 while the internal adds to the substance of the stalk. The 

 central stalk tissue is a dense plexus of filaments which anasto- 

 mose freely with each other. In Desmarestia a sort of mantle 

 or covering of the original axis is produced by filaments which 

 grow from the cells of the lateral branches originating just 

 below the growing point, which become united together and to 

 the original axis, subsequently undergoing differentiation with 

 cortical and internal tissues. 



With the exception of the lowest group the colouring matters 

 are associated with definite chromatophores, which may occur 

 singly or in numbers in the cells. The pigments which are 

 formed are phycoerytJirine in the red, phycoxanthine and 

 jyliycojyhceine in the brown, Siud phycocyanine in the blue-green 

 Algae. Besides these, all the members of the group contain 

 chlorophyll, which is the only colouring matter in the green 

 forms. The other pigments may be extracted by fresh cold 

 water, which dissolves all but the chlorophyll. The disposition 



