IV. LAMINARIACEiE. 81 



but in some delicately membranaceous ; the internal structure fibroso-cellular, the 

 flesh being chiefly composed of interlacing threads, formed of strings of cylindrical 

 cells, placed end to end. The plants of this Order are almost all of large, 

 frequently of gigantic size, either tubular or furnished with a stipe which expands 

 at the summit into a leafy frond. In the least developed genus (Adenocystis) the 

 frond consists of a hollow, membranous bag, contracted at the base into a little 

 stalk, and gradually tapering to the apex into a simple point. At the next stage 

 {Chorda) the form is still tubular, but the tube becomes cylindrical, or filiform, and 

 is divided internally into several compartments, by transverse membranes stretched 

 across its cavity. In the more perfect genera we clearly recognise a cylindrical 

 solid stem or stipe, occasionally vesicular in its upper portion, and bearing at its 

 summit an expanded leaf. This stem is in most cases simple ; in the most perfect 

 genera alone it becomes branched, its divisions being repetitions of the primary 

 idea. The leafy expansion crowning the stem or branches is sometimes ribbon- 

 shaped, quite simple and tapering to its extremity ; sometimes it is cloven verti- 

 cally into many narrow lacinise, by a process of natural splitting which takes place 

 in a very irregular manner ; sometimes it is regularly pinnatifid (as in Ecklonia) 

 and lastly (in Agarum and Thalassiophyllum) the whole expansion is perforated 

 with holes, like a sieve. In the majority of cases the leaf is ribless ; but in the 

 more fully organized a midrib, formed of a prolongation of the apex of the stipe, 

 traverses its substance. Air-vessels are very often wanting ; where they are found, 

 they are formed either by distensions of the upper portion of the stipe, or (in Ma- 

 crocystis) by vesications of the petioles of the leaves. 



In those species that are perennial the stipe lasts for several years, but the leaf is 

 changed at the end of each season. The process for effecting a change of leaf is 

 tJ-radual, and commences long before the fall of the previous leaf. The ncAV leaf is 

 not formed, however, in the axil of the old one, but begins at the apex of the stipe, 

 or in that portion where the stipe, or common petiole, passes into the leaf. At 

 that point, new and vigorous tissue is always found ; there a new lamina begins to 

 expand, and as it elongates it gradually pushes before it the older part of the leaf, 

 which for a long time adheres to the apex of this new part, and falls away only 

 when the new leaf has reached the normal size. 



The fructification of this Order is on a very simple type of development. Innu- 

 merable minute spores, each contained within a hyaline perispore, are formed out 

 of the surface cells either of the whole frond, or of some large and imperfectly 

 defined portions of it. In the highest types only (as in Alaria) are spores found 

 in spaces definitely limited, or in proper leaflets. In the lowest ( Chorda) they clothe 

 the whole surface, and in most other cases {Laminaria, Agarum, S,-c.) they form 

 cloud-like, dark-coloured patches of considerable extent and uncertain limits. 

 Usually but one spore is found in each perispore, but in some each perispore con- 

 tains four sporules. Barren filaments, or paranemata, occasionally accompany the 

 spores, and in some cases Antheridia are found attached to them. These last are 

 oval cells, filled with minute corpuscles. 



The Laminar iacece, though formed on a much less fully organised type than the 



VOL. III. ART. 4. ^^ 



