THALLOPHYTA. 647 



ranea: the lower part of the long cylindrical stalk is incrusted with calcium car- 

 bonate, and fixed to its substratum by short irregularly branched rhizoids. The 

 rhizoid-bearing portion is called the foot, and below it there is a thin-walled branched 

 continuation of the stalk, called the basal division. Near the apex of the stalk are 

 borne 1-4 whorls of polychotomously branched sterile appendages, which soon 

 fall off. 



Above these is an umbrella-shaped whorl of simple appendages in lateral contact, 

 wnose cavities are not shut off from that of the stalk. The whole of the upper part 

 of the plant dies off each autumn, only the foot and basal division remaining alive 

 through the winter. In the spring a new shoot is produced. Apparently after 

 several years the contents of each simple appendage of the umbrella (which may 

 now be a centimetre or more in diameter) divides up into a number of oval bodies, 

 each surrounded by a fairly thick wall and containing chlorophyll and starch. 

 These are the gametangia. After their escape by the dissolution of the umbrella, 

 the contents of each divides up to form a number of gametes. Considerable pressure, 

 caused by swelling of the ectoplasm and osmotic tension in the vacuole of the game- 

 tangium, bursts off a Hd at one end, and the gametes escape. Conjugation only 

 occurs between gametes derived from distinct gametangia. 



Basycladeoe. — No distinction between fertile and sterile appendages. Dasycladus 

 has a single stalk -cell fixed below like Acetabularia, but bearing very numerous 

 whorls of appendages, which stand so close together as to give the entire plant 

 a resemblance to a minute fox's brush. Each appendage bears a terminal whorl 

 of branches, and in the middle of these is a shortly-stalked, nearly spherical 

 gametangium. The gametes conjugate, but apparently only with those from 

 certain other plants. This fact at first led to the supposition that we had here 

 a physiological distinction of sex in gametes, which in external appearance are all 

 alike. This is, however, quite an unjustifiable and unnecessary assumption. We 

 have no right to predicate sexual differences between gametes which do not show 

 any of the well-recognized characters of male and female reproductive cells. The 

 tendency to avoid pairing with closely related gametes, which we may call exogamy, 

 is quite a distinct phenomenon, not only among isogamous Algse, but also among 

 many of the higher plants, where it coexists with strongly -marked sex. The pheno- 

 mena of self -sterility is an extreme case of this. 



Neomeris and Cymopolia are two tropical and subtropical genera, whose thallus 

 is very strongly incrusted with calcium carbonate. The arrangement of the 

 branches resembles that found in Dasycladus, but on the ends of the younger ones 

 hairs are borne, which serve to protect the growing apex of the plant. In Cymo- 

 polia, of which the main stalk branches, and the thallus attains a considerable size, 

 these hairs are borne by simple branches produced on special constricted and uncal- 

 cified zones of the stalk. The apices of the secondary branches are in both genera 

 swollen up, and in close lateral contact, thus forming a continuous surface on the 

 exterior of the plant. The calcium carbonate is deposited as a thick layer under- 

 neath these swollen ends. 



