Xii INTRODUCTION. 



expands into broad flat fronds. Only the higher tribes show 

 any distinction into stems and leaves, and even in these, 

 what appears a stein in the old plant, has already served, at 

 an earlier period of growth, either as a leaf, as in Sargas- 

 sum and Cystoseira, or as the midrib of a leaf, as in Deles- 

 seria. A few exhibit leaves or flat fronds formed of a 

 delicate, perforated net-work, resembling fine lace or the 

 skeletons of leaves, a structure which is also found among 

 zoophytes. Of those so constructed, the most remarkable 

 are the New-Holland genus Thuretia, the East-Indian 

 Dictyurus (Callidictyon, Grev.), and Martensia, a genus 

 lately discovered at Port Natal, in South Africa, by Dr. 

 Krauss, which produces fan-shaped fronds, the lower half 

 of which has the structure and colour of Nitophyllum, the 

 upper that of the delicate net-w r ork of Thuretia. Claudea, 

 one of the most singular of all the Algae, has a reticulated 

 frond of somewhat different structure. In this plant the 

 phyllodia are not formed by the simple interlacement of 

 fibres, or the perforations of a membrane, but by the anas- 

 tomosing of minute, ribbed leaflets, which are at first free, 

 and gradually connect their points with the rib of the leaf- 

 let next above them. The new originate all at the upper 

 side of the older leaflets, and stand at right angles with 

 them; thus a net-work is soon formed, and as the distances 

 between the leaves are small, the structure is delicately 

 lacy. 



Among British Algae, the only structure analogous to 

 these exists in Hydrodictyon, a fresh water Alga, which 

 grows in the form of a perfect net, with regular meshes. 



The substance of which the frond consists is as variable 

 as the form. Some are mere masses of slime or jelly, so 

 loose that they fall to pieces on being removed from the 

 water ; others resemble, in feel and appearance, threads of 

 silk ; some are stiff and horny ; others are cartilaginous, 



