IV. 
LAMINARIACEiE. 
81 
but in some delicately membranaceous ; tlie internal structure fibroso-cellular, tlie 
flesli 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 laciniie, 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 Thalassiopliyllum) 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 
gradual, and commences long before the fall of the previous leaf. The new 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 aAvay 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 , <fyc.) 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 Anther idia are found attached to them. These last are 
oval cells, filled with minute corpuscles. 
The Laminariacece , though formed on a much less fully organised type than the 
VOL. in. art. 4. m 
