142 THE ALGAE 



The swarmers from these gametangia are thought to be male 

 gametes. Besides these organs there are larger and spherical uni- 

 nucleate monosporangia borne on a stalk cell and partly immersed 

 in the branches. These at one time were thought to be oogonia but 

 at present they are regarded as an accessory means of reproducing 

 the sexual generation. The asexual plant reproduces by means of 

 quadrinucleate spores formed singly in stalked or sessile, terminal 

 or intercalary, monosporangia. Meiosis has been reported as occur- 

 ring in these sporangia and this would be expected if they were 

 primitive tetraspores. It would seem that the plants known as 

 Haplospora glohosa and Scaphospora speciosa are simply alternate 

 phases of one and the same species, the latter being the sexual 

 generation. The whole life cycle, however, is in need of detailed 

 study. 



CUTLERIALES 



This order is characterized by trichothallic growth, regular 

 alternation of generations, and a well-marked anisogamy which in 

 some respects approaches oogamy. They are generally placed in 

 the Isogeneratae, even though this leads to a difficulty because in 

 Cutleria the two generations are not equal morphologically al- 

 though they are equal in Zanardinia. In Micro zonia only the dip- 

 loid generation is known at present. On this classification, therefore, 

 Cutleria must be regarded as a modified member of the Isogenera- 

 tae or else it must be separated from Zanardinia and put in a sepa- 

 rate family in the Heterogeneratae. A solution to this problem is to 

 regard both types of alternation as having arisen from an ancestral 

 form that was heterotrichous (see p. 316). 



CuTLERiACEAE : Cutleria (after Miss Cutler). Fig. 79 



The gametophyte and sporophyte generations are distinctly 

 heteromorphic and also differ in their seasonal occurrence, the 

 former being a summer annual whilst the latter is a perennial reach- 

 ing its maximimi vegetative phase in October and November with 

 a peak fruiting period in March and April. The gametophyte is an 

 erect, flattened thallus with irregular branching brought about by 

 periodic failure of the marginal filaments to fuse together. In cross- 

 section the plant consists of larger cells in the centre and smaller 

 ones in the epidermis. The thallus and apices are clothed with 



