EUPHYCOPHYTA 187 



South Pacific Ocean and near the Cape of Good Hope, all being 

 regions where the temperature of the water ranges between o and 

 20° C. The upper part of the plant floats on the water surface, kept 

 there by the basal bladders of the leaves. In the juvenile plant the 

 stipe is simple and solid, but later on it branches one to three 

 times in a dichotomous fashion, although ultimately the branching 

 becomes unilateral and sympodial. The growing region on each 

 branch is ventrally situated in the terminal blade, and it is here that 

 splitting takes place to form the individual laminae. The spHtting 

 is brought about by local gelatinization of the inner and middle 

 cortex together with a cessation of growth in the epidermal area; 

 this forces the adjacent tissues into the gelatinized areas until 

 finally the epidermis is ruptured. Of the two segments formed in 

 this splitting process, one remains arrested, whilst the other, the 

 outer, continues growth. The internal structure of the stipe and 

 fronds is typical of the order as described under Laminaria (see 

 p. 175). The rate of growth, during the growing season, of this 

 plant is quite spectacular. On the Pacific coast of North America 

 it has been found to be of the order of y-i cm. ±4-3 per day, which 

 works out at about ij blades daily. The amount of organic matter 

 acamiulated in the growing tip is equivalent to that produced and 

 exported from 35 blades immediately below. There are very many 

 more blades than this in the plant during most of its life, so that the 

 remaining surplus of photosynthesis over respiration must pre- 

 sumably be translocated to the stipe and haptera and stored as 

 reserve. 



Macrocystis (and also Nereocystis) is interesting in that in the 

 cortex there are sieve tubes as well as trumpet hyphae in the 

 medulla. Neighbouring tubes are connected to each other by 

 lateral filaments, the cross walls of which are also perforated. Re- 

 production is by means of zoospores produced on sporophylls, 

 generally dichotomously branched and with or without a bulb, near 

 the base of the plant. Although two kinds of zoospore have been 

 recorded, recent work has failed to substantiate the statement. 

 Enormous numbers of zoospores are produced, the actual number 

 being related to the size of the frond. Each sporangium produces 

 thirty-two swarmers and on a large frond as many as 693,760 zoo- 

 spores may be liberated per square millimetre of frond (both sur- 

 faces). On germination the zoospores give rise to the characteristic 

 laminarian gametophytes, which are relatively long-lived (six 



