LAMINARIALES (HETEROGENERATAE) 175 



canals become more and more submerged in the thallus. The 

 attachment organs or crampons, which are positively geotropic, 

 have an apical growth and differ from the rest of the thallus in that 

 there are no connecting hyphae nor is there any pith. The amount 

 of conduction necessary in these plants would be expected to be 

 small, but even so the degree of differentiation is remarkable. So 

 far as the lamina is concerned the group is usually regarded as 



20-1 



15 



10- 



5- 



sterile ^ aLt$ sterile 



DEC. JAN. FEB. 



Fig. 118. Laminaria. A, L. digitata, marked thallus before growth in summer. 

 B, L. digitata, marked thallus after growth in summer. C, effect of temperature 

 on fruiting of gametophytes in L. digitata. (After Schreiber.) 



primitive because the new portions do not originate separately but 

 by intercalary growth from an existing portion (cf. fig. 115). 



The sporangia and paraphyses are borne in irregular or more or 

 less regular sori on both sides of the lamina. It is probable that the 

 zoospores possess an eye-spot, but it must be very small because 

 in the three species where it has been recorded it was very 

 difficult to distinguish. The zoospores, which in one or two cases 

 are reported to be of two sizes, germinate to form minute gameto- 

 phytes, but on germination they first put out a tube that terminates 

 in a bulbous enlargement into which the contents of the zoospore 

 migrate. There the nucleus divides and one daughter nucleus passes 



