INTRODUCTION TO CRYPTOGAMIC BOTANY. 169 



be determined after a series of well-directed observations and 

 experiments, instituted for the express determination of the 

 point. It was before stated, how descending joints after the 

 manner of roots form a cellular coating to the simple stem in 

 the genus Batrachospermum. Precisely the same process takes 

 place in Callithamnion (Fig. 26 his), insomuch that a section 

 at a given point which would have formerly shown a single 

 cell, exhibits such a cell, surrounded by a circle of others. In 

 many instances, however, as in Polysiphonia, the external 

 cells are from the first an essential part of the plant, and from 

 this the transition is easy to such genera as Rytiphlwa and 

 Geramiu7n, and from thence to the most complicated fronds. 

 Others, again, are formed after the fashion of Ulvce, by a simple 

 expansion of a frond, consisting of one or two layers of cells. The 

 accession of veins and ribs gives additional thickness, and we 

 have thus again a way paved to more complicated leaf-like 

 structures like that of Wormskioldia sanguined. The ex- 

 quisite colour, and the variety of forms, often of extreme 

 elegance, simulating the most delicate leaves, or mosses, with 

 infinite interchange of ornament in the form of teeth, fringing 

 hairs, net-work, &c., are attractive to the commonest observer, 

 and have ever made their collection an object of interest to 

 those whom search of health or amusement carries to the 

 coast. In a systematic point of view they recommend them- 

 selves to notice by pretty certain indications of sexual distinc- 

 tions, at least in numerous genera, and by the double form 

 under which the reproductive bodies appear in almost every 

 genus. The one consists of tetraspores arising from the 

 division of a single endochrome in certain privileged cells, and 

 variously situated, always in distinct individuals ; the other, in 

 a collection of organisable tissue, more or less distinct from the 

 general mass, sometimes free, sometimes immersed ; consisting 

 of a number of articulated threads in distinct hollow concep- 

 tacles, or wart-like tubercles variously situated, the joints of 

 which are to a greater or less extent transformed into spores, 

 sometimes every joint, sometimes one or two only, whether 

 terminal or central, with a mixture sometimes of barren threads ; 

 of similar threads immersed in the substance of the frond, with- 



