136 THE ALGAE 



which form characteristic features. Structurally they can be re- 

 garded as strengthened multiseriate filaments, Sphacella perhaps 

 being one of the more primitive members of the group with a non- 

 corticate monosiphonous axis. 



In most members of the order two types of lateral branch occur, 

 those of limited and others of unhmited growth. The plants exhibit 

 heterotrichy with a well-developed prostrate system, which is 

 perennial and from which new erect systems can arise each season. 

 In those cases where it has been studied there is an alternation of 

 isomorphic generations. The order is most vddely represented in 

 the southern hemisphere, and it may well have originated in the 

 Australasian region. 



* Sphacelariaceae: Sphacelaria (gangrene). Fig. 75 



The plants grow attached to stones or other algae by means of 

 basal discs or rhizoids that have spread down from the lower cells 

 of the axis. The basal disc may give rise to horizontal stolons and 

 so enable the plant to spread. 



The erect system is filamentous and the type of branching is 

 termed hemihlastic in which the laterals, whether of definite or 

 indefinite grov^oh, arise from the whole height of a cell representing 

 the upper segment derived from the division of a primary segment 

 (Fig. 75, E). If the branching is pinnate practically every superior 

 segment produces branch initials, though in some cases the initials 

 may remain dormant and the branching is then less luxuriant. 

 Branches of limited growth bear colourless hairs which are cut off 

 as small cells to one side of the apical cell (Fig. 75, B-D). As the 

 apical cell elongates the hair comes to occupy a lateral position. 

 These hairs may disappear with age. This type of hair formation is 

 regarded as sympodial, the hair initial being the true apical cell 

 whilst the so-called branch axis is regarded as a subsidiary branch. 

 Branching, which takes place in this way is said to be holohlastic. 



Growth in length takes place by horizontal divisions of the 

 apical cell, and when the basal cell so cut off (S in Fig. 75 E) has 

 reached a certain length it divides into a superior and inferior seg- 

 ment. These segments exhibit no further enlargement, though 

 they may divide by longitudinal and transverse walls to form a 

 corticated thallus. This type of vegetative system is said to be 

 leptocaulous. 



Vegetative reproduction takes place in this genus by means of 



