INTRODUCTION. 



13 



artificial green may be imparted to the specimens ; but such a mode of preparation 

 of specimens ought never to be practised by botanical collectors, though it may 

 sometimes serve the purpose of makers of seaweed pictures. 



THE FRUCTIFICATION 



of the Algaj may be more conveniently described in the systematic portion of this 

 work, when speaking of the various forms it assumes in the different families. I 

 shall at present, therefore, limit myself to a very few general observations. The 

 spore or reproductive geramule of the Algaj is in all cases a simple cell, filled with 

 denser and darker coloured endochrome (or colouring matter) than that found in 

 other cells of the frond. In the simplest Algse, where the whole body consists of a 

 single cell, some gradually change and are converted into spores, without any 

 obvious contact with others: but far more frequently, as in t\\Q Desmidiacecs and 

 Diatomacece,a spore is formed only by the conjugation of two cells or individual plants. 

 When these simple vegetable atoms are mature, and about to form their fructification, 

 two individuals are observed to approach ; a portion of the cell-wall of each is then 

 extended into a tubercle at opposite points; these tubercles come into contact 

 and at length become confluent ; the dissepiment between them vanishes, and a 

 tube is thus formed connecting the two cavities together. Through this tube the 

 matter contained in both the old cells is transmitted and becomes mixed ; changes 

 take place in its organization, and at length a sporangium or new cell filled 

 with spores is formed from it, either in one of the old cells, or commonly at 

 the point of the connecting tube, where the two are soldered together. Then 

 the old empty cells or plants die, and the species is represented by its sporan- 

 gium, which may remain dormant, retaining vitality for a considerable time, 

 as from one year to another, or probably for several years. These sporan- 

 gia, which are abundantly formed at the close of the season of active growth, 

 become buried in the mud at the bottoms of pools, where they are encased 

 on the drying up of the water in summer, and are ready to develop into new fronds 

 on the return of moisture in spring. 



Many of the lower Alga? form fruit in this manner, to which the name con- 

 jugation is technically given. The thread-like Silk -weeds of ponds and ditches 

 {Zygnemata and Mougeotice, &c.) are good exainples of such a mode of fruiting. In 

 these almost every cell is fertile, and when two threads are yoked together, a series 

 of sporangia will be formed in one thread, while the other will be converted into 

 a string of dead, empty cells. Before conjugation there was, seemingly, no differ- 

 ence between the contents of one set of cells and of the other ; so that there is no 

 clear proof of the existence of distinct sexes in these plants, however much the 

 process of fruiting observed among them may indicate an approach to it. 



The process of fruiting in the higher Algte appears to be very similar : namely, 

 spores or sporangia appear to be formed by certain cells attracting to themselves the 

 contents of adjacent cells ; and in the compound kinds empty cells are almost always 

 found in the neighbourhood of the fruit cells ; but with the complication of the 

 parts of the frond, the exact mode in which spores are formed becomes more diffi- 



