470 ^ " Colonial " Herhariuvi Specimen. [Sess. 



labelled " Istlimia," and is thus intended to be classified with 

 diatoms, — but a very casual inspection discloses the fact 

 that it illustrates more than one organised form. The first 

 glance conveys the impression that it is meant to go with the 

 sea-weeds, seeing that what is actually fixed to the paper, and 

 is conspicuously the seat of the " colony," is one of the red 

 algse, Delesseria alata. First let us look at this most pro- 

 minent element of our specimen. In all probability the 

 Delesseria grew on some larger sea-weed, from which it was 

 forcibly removed by a storm, or by the hand of the sure- 

 footed prying collector. Its position on another sea-weed 

 does not imply parasitism, even although its tissues are 

 intimately in conjunction with those of the obliging host. 

 Commensalism, of which this is an example, is very common 

 among marine algaj, and very striking in many cases it is. 

 Our Delesseria is one of the great family of the red sea-weeds, 

 so named on account of their colour, which is due to the 

 presence of a substance called phycoerythrine. The group 

 includes those with shades of colour varying from rose to 

 violet. The red colour-stuff can be dissolved out by cold 

 fresh water, in which it will appear carmine-red in transmitted 

 light, and yellow or green in reflected light. It must be 

 borne in mind that chlorophyll is also present in the plant, 

 but that it is so mixed with and concealed by the red material 

 as to be invisiljle till the latter is removed. Our species 

 branches copiously in a dichotomous and interruptedly pinnate 

 fashion to form a bilateral thallus. Thickened mesial and 

 oblique lines look like midribs and nerves, but of course these 

 are not to be thought analogous to the structures so termed 

 in an ordinary leaf, seeing that in our sea-weed there are no 

 vascular tissues such as go to form the strengthening elements 

 of higher plants. Two methods of propagation exist in 

 Delesseria. One is by tetraspores, which are developed in 

 sporangia occurring imbedded irregularly in the thallus along 

 the midrib of the end segments or terminations of the pinnae, 

 and on special small " leaflets." The tetraspores are not 

 endowed with means of locomotion, and are dependent on cur- 

 rents for their distribution. The other method is by carpo- 

 spores, which develop in special receptacles — cystocarps — 

 as a consequence of fertilisation by spermatia. The cysto- 



