94 THE ALGAE 



carbonate is deposited wherever there is a mucilage layer and an 

 aggregation of the chloroplasts, but apparently both these condi- 

 tions must be fulfilled before Hme can be laid down. The principal 

 interest of this form lies in its morphological resemblance to certain 

 fossil genera (cf. p. 301). The species are dioecious and sexual re- 

 production is isogamous. In this genus the primary nucleus changes 

 shape before fragmenting at gamete formation (Fig. 50). 



Dasycladaceae: Acetahularia {acetabular little cup; aria, derived 

 from). Fig. 53 

 This is a Hme-encrusted genus which is confined to warm waters, 

 extending up as far as the Mediterranean in the northern hemi- 

 sphere. The plants consist of an erect elongate axis bearing one or 

 more whorls of branched sterile laterals with a single fertile whorl 

 at the apex. The sterile whorl or whorls are frequently shed in the 

 adult plant leaving a mark or annulus on the stem to show where 

 they were formerly attached. The fertile whorl is composed of a 

 series of long sac-like sporangia which are commonly fused, though 

 they are sometimes separate: these are borne on short basal seg- 

 ments which are morphologically equivalent to the primary 

 branches. The basal segments also bear on their upper surface small 

 projections, with or without hairs, which form the corona, whilst 

 in one section of the genus there is also an inferior corona on the 

 lower surface. In A. mediterranea two or three years elapse before 

 the plant attains to maturity. In the first year the branched hold- 

 fast produces an upright umbiHcal thread, together with a thin- 

 walled, lobed outgrowth that penetrates the substrate in order to 

 function as the perennating organ. The aerial part dies, and in the 

 next year or years a new cylinder arises that bears one or more 

 sterile whorls of branches, until in the third or even a later year, a 

 shoot develops which produces one deciduous sterile whorl and a 

 single fertile whorl or umbrella. Each sac-like sporangium, or 

 imibrella lobe, gives rise to a number of multinucleate cysts which 

 are eventually set free through disintegration of the anterior end of 

 the sporangium. In the spring biflagellate isogametes are liberated 

 from these cysts and fuse in pairs, or else develop parthenogene- 

 tically. Use has been made of the uninucleate condition to deter- 

 mine the effect on morphology of varying ratios of nuclei of 

 different species. This is possible because part of one species can 

 be grafted onto another. In particular A. crenulata and A, mediter- 



