INTRODUCTION TO CRYPTOGAMIC BOTANY. 205 



simple, when it is not to be distinguished from the Mediterranean 

 iV. luhricurifh. Belesseria alata is sometimes quite destitute 

 of margin, when it becomes D. angustissima. Several species, 

 which in the ordinary condition have the tips of the fronds 

 perfectly straight and even, occasionally produce hooked clasp- 

 ing tips. Cystoclonium purjnirascens, produces such a variety 

 on our coasts, when it looks like a Hypnea, to which genus 

 it has consequently been referred, and Delesseria lacerata 

 sometimes forms an inextricable mass of intricate fronds, 

 attached to each other so closely that they cannot be separated 

 without rupture. 



184. The causes which produce such changes are in general 

 unknown, though sometimes we have a slight clue to variations. 

 Chondrus crispus, when exposed to the fresh water of the 

 estuary, acquires great thickness and breadth ; while at low- 

 water mark it is thin with forked narrow branches.* Multi- 

 tudes of intermediate forms occur of great variety of outline 

 and thickness, so that in these respects and general habit it 

 almost rivals GelidiuTn corneum. Dumontia Jiliformis 

 becomes singularly crisped and thick, where a strong tidal 

 current comes down, instead of having tolerably even slender 

 tapering branches. Dasya coccinea in deep water seldom 

 produces fruit, and is very irregularly branched. The squar- 

 rose variety, which is obtained by dredging, has most of the 

 branches set on at a right angle, so as to make it appear at 

 first sight totally different. Intermediate forms, however, are 

 found by dredging on different bottoms, which afiford a com- 

 plete connecting series. 



185. In some cases, where the circumstances appear just the 

 same, one species uniformly replaces another without any inter- 

 mixture, Callithamnion spongiosum, for instance, takes the 

 place of C. arbuscula on shores where the latter is not found, 



* Some species of Algae, as Bangia atroptorpiorea, grow indifferently 

 in fresh and salt water, without any perceptible diflVencc. Tlie nearly 

 allied Bangioid form of Porplu/ra vulgaris is most iiermanent where it 

 is veiy near high-water mark, and therefore gets less nourishment. It 

 is evident, however, that the largest forms of true Ehodosperms do not 

 unifoi-mly occur where they are most constantly immersed. 



