THE MARINE ALG^E OF NEW ENGLAND. 13 



though the species included in this large order differ from one another in 

 size and habit to an extent that would certainly forbid their being placed 

 together, if we considered merely the character of the frond, yet they re- 

 semble one another very closely in their mode of reproduction, which is 

 accomplished by means of zoospores. The Zoosporece are divided into four 

 suborders, the Chlorosporew, or Chforozoosporece, as the name is sometimes 

 written, the Phccosporecv, or Phceozoosporea, the Bryopsidece, and the Bo- 

 trydiece. The former are abundant in both fresh and salt water. They 

 especially frequent brackish waters and high tide-pools. The mass of the 

 vegetation in brackish rivers is formed of species of this order. The species 

 are either filamentous or else in the form of green membranes, as in the sea- 

 lettuces, Ulvtf, which abound in muddy places between tide-marks. The 

 contents of any of the cells may be transformed into zoospores, which 

 escape from the mother cell usually at daybreak. The zoospores are of 

 two kinds, microzoospores and macrozoospores. The latter are produced 

 few in number in the mother cell, and when they have escaped into the 

 water they are seen to be furnished with four cilia placed at one end, 

 and with a dark red spot on one side. After swimming about for a 

 short time they come to rest, the cilia disappear, a wall of cellulose is 

 formed around the zoospore, which then begins to divide and produce a 

 plant like that from which it came. The microzoospores are' borne in 

 considerable numbers in the mother cell, and when they escape they are 

 seen to have only two cilia at one end, and a dark red spot on the side. 

 The microzoospores, after swimming about a short time, approach one 

 another in pairs, occasionally in threes, which in a short time coalesce 

 so as to form a body known as the zygospore, or, to use a term first ap- 

 plied by Eostafinski, the isospore, which has four cilia and two dark red 

 spots. The zygospore swims about for a short time, then comes to rest, 

 takes on a cellulose wall, and begins to divide in the same manner as a 

 macrospore. This process of union is called conjugation, and represents 

 sexuality in its lowest form, it being impossible to say which of the con- 

 jugating bodies is male and which is female. It is only the microzoo- 

 spores which come from different mother-cells which conjugate, but it is 

 not quite certain whether the cells must belong to different individuals. 

 The microzoospores, however, do not always conjugate. More frequently 

 they do not, but, after swimming about separately for a short time, lose 

 their cilia and begin to grow just like the macrozoospores. If one wishes 

 to examine the zoospores, he has only at evening to put a piece of sea- 

 lettuce into a vessel of salt water, and at daybreak the zoospores will 

 have formed a green cloud in the water. If the cloud consists of mi- 

 crozoospores, it will collect in the vessel on the side nearest the light ; 

 if composed of macrozoospores, on the side away from the light. Con- 

 jugation was first observed in a marine species ( Ulva) by Areschoug, 

 but had previously been observed by Pringsheim in a fresh- water spe- 

 cies (Pandorina). Since then conjugation of zoospores has been studied 

 by several observers. 



