30 FIRST GROUP.- -THALLOPHFTES. 



more than thirty centimetres long, with a number of spherical nuclei in the protoplasmic 

 layer on the cell-wall. In Biyopsis the main axes bear secondary axes pinnately 

 disposed, with limited power of growth. The lowest of these are thrown off after 

 their cavity has been separated from that of the main axis by the formation of a 

 gelatinous stopper, such as is not unfrequently to be observed in the Siphoneae. 

 Caulerpa, on the other hand, has a tolerably thick creeping main axis, which in the 

 case of C. prolifera, a not uncommon plant of the Mediterranean, bears branched 

 rooting structures on its ventral surface, branches on its lateral surface, and on its dorsal 

 surface broad leaf-like formations. Though often several metres long, it is not divided 

 internally into compartments by cell-walls, but forms one continuous cavity, which 

 is however crossed by bands of cellulose stretching from wall to wall to strengthen 

 the thallus. The same purpose is attained in other species, whose thallus is an 

 unusually long thin-walled and much-branched tube, by the branches being densely 

 interwoven, the result being an algal body of some size with the appearance of 

 true tissue-formation. This is the case with Codium, Udotea and Halimeda, the 

 latter of which we may take as an example of the rest. This plant with the 

 habit of an Opuntia (Fig. 13 A} appears to be formed of a number of separate 

 parts with narrower bases all strung together, and with their outer surface more 

 or less incrusted with lime. The longitudinal section (Fig. 13 Z>) however shows 

 that the whole vegetative body is here also one branched unsegmented tube. 

 The branches are especially crowded together towards the circumference, and thus 

 form the compact outer layer of the thallus. In C odium, which has a similar 

 formation of its tissue, the tubes swell out at the surface and form a palisade- 

 like layer. These tubes are often separated from the inner filaments from which 

 they spring by a diaphragm a strongly refractive stopper, which first appears as 

 a solid annular thickening on the wall and grows continually towards the inside 

 till it closes the lumen of the tube. Another subdivision of the Siphoneae is 

 distinguished by the verticillate arrangement of its branches, the further ramifi- 

 cations of which form umbels. This is the case with Dasydadtis and Acetabularia. 

 The latter resembles externally a small mushroom ; on a thin lime-incrusted stalk 

 is perched a small umbrella-like cap, which is divided into a large number of 

 separate filaments by radiating walls that do not however quite reach the centre. 



The development of Acetabularia is well known through the investigations of De 

 Bary and Strasburger 1 , and will serve as an example of the development of the isoga- 

 mous Siphoneae. An important part of the plant in reference to the manner of growth, 

 and one which has not yet been mentioned, is the basal portion first noticed by De Bary. 

 The stalk has tuber-like organs of attachment at its lower extremity, but it rises from 

 among them in the shape of a thin-walled vesicle with branches usually in the form of 

 lobes. Acetabularia is a perennial plant, but each stalk with its cap is only annual. The 

 cap with the upper part of the stalk dies at the end of its period of vegetation, and the 

 lower part only remains ; but the protoplasm retreats into the basal portion, especially 

 that which was spoken of above as the vesicle, and fences itself off above by a transverse 

 wall. In this condition the plant passes the winter. When the next period of vegeta- 

 tion begins, the transverse wall arches outwards and developes into a cylindrical tube 

 growing out of the remains of the old plant, and this tube ultimately assumes the form 

 of a cap-like shoot. This process is repeated during a series of years, probably till the 



1 De Bary u. Strasburger, Acetabularia incditcrranca (Bot. Ztg. 1877, p. 713 ff.). 



