V. RHODYMENlACEiE. 145 



liform, branching strings issuing from a placenta, at length massed together with- 

 out order. 



Natural Character. Root mostly a mere disc, sometimes, as in Plocamium, 

 branching. Frond membranaceous in substance, very variable in general habit and 

 branching. Most commonly the membrane is flat, narrow, or expanding into broad 

 dichotomously or irregularly cleft expansions, without trace of midrib or veins. 

 In one genus only ( Wormskioldia) nerved leaves, of definite form and delicately 

 membranous areolated substance, occur. In Plocamium the frond is linear, often 

 very narrow, much branched in a pinnate order, flat or compressed, with or without 

 midrib. In Rhahdonia it is terete, the exterior strata composed of the ordinary 

 polygonal cells, the axis of closely interwoven cylindrical cells disposed in fila- 

 ments. 



The sporiferous nuclei are always lodged in proper conceptacles, external or 

 partly immersed, usually hemispherical and destitute of apical pore, sometimes open- 

 ing at the apex, or even furnished with a prominent orifice. These conceptacles 

 are either marginal or scattered over the surface, or (rarely) formed in proper 

 leaf-like processes. "Within a densely cellular pericarp, a basal or central placenta 

 is often largely developed ; from it issue toward all sides dense tufts of branching 

 spore-threads, either united in a single nucleus or divided into several, which are 

 sometimes separated by barren filaments running from the placenta to the pericarp. 

 The sporiferous threads are moniliform, branched, articulated, each articulation 

 containing at an early stage a simple mass of endochrome, but by repeated cell- 

 division the contents of each cell is finally converted into a cluster of spores, 

 held together by the dilated cell-wall. The clusters thus originated, being con- 

 fined within the narrow cavity of tlie conceptacle, are closely pressed together and 

 at length massed into nuclei or nucleoli without ob\'ious order. The spores, also 

 by reason of pressure, become irregularly angular or wedge-shaped. The tetra- 

 spores are roundish or oblong, and variously parted ; and are either dispersed 

 among the surface cells, or collected in definite sori, or lodged in proper leaflets. 



This order has recently been pi:oposed by Professor J. G. Agardh to include a 

 few genera which, on account of the very diflferent structure of their conceptacular 

 fruit, he has rejected from the Sjihcerococcoidew ; a measure rendered necessary by 

 the new principles of arrangement developed by that author. These plants, 

 however, so closely resemble the genuine Sphcerococcoidece in external habit, and 

 even in the internal structure of the stem and leaves, that recourse must some- 

 times be had to an accurate microscopic analysis of the contents of the concepta- 

 cle, before the student can ascertain the proper place in the system of the plant 

 under examination. This is notably the case in the genera Wormskioldia (founded 

 on Delesseria sanguinea, Ag.) and Delesseria {D. sinuosa, ^'c), tAvo genera with 

 fronds of precisely similar texture and appearance, but Avith a fructification of a 

 structure so dilFerent that we are compelled to jjlace them not only in different 

 Orders, but in different Series. If this seems an unnatural distribution, as it cer- 

 tainly is contrary to long established prejudice, let it be remembered that there are 

 species of Cactus, Sfapelia, and Euphorbia equally resembling each other in habit, 



VOL. IV. ART. 5. u 



