STRUCTURE, ETC. OF THE A.L&M. 139 



direction, succeeding each other in a line, while in 

 those plants whose parts are broad and extended, 

 the spores or cells multiply laterally as well as 

 longitudinally, and thus the broad frond is formed. 

 Why it is that one species always increases in one 

 direction, and others in a different direction, is 

 one of those mysteries the cause of which eludes 



our investigations. 



It is worthy of remark that the reproductive 

 power of the algae differs considerably in different 

 kinds. Thus in the ulva already spoken of, 

 known as sloke, whose thin grass green or olive 

 brown and purple fronds abound on some of our 

 shores, every part of the frond is capable of pro- 

 ducing cells or spores, and thus the plant is capable 

 of spreading indefinitely on every side. It is 

 otherwise with the alga3 of the higher orders, 

 such as the bladder wrack or Fucus vesiculosus, 

 and the notched wrack, F. serratus ; in these it 

 has been discovered that the tissue is less uniform 

 in character than in the simpler forms of marine 

 vegetation, and that some approach is made to 

 that separation of organs perceptible in the higher 

 orders of the Cryptogamia. Unlike the ulva, the 

 fuci now mentioned have their reproductive 

 powers restricted to one part only, the extremity 

 of the fronds. These are found to dilate them- 

 selves into receptacles or spore-sacs, beautifully 

 adapted to their office, being furnished with pores 

 through which the germs make their escape when 

 arrived at maturity. 



There is another peculiarity in the structure of 



