Ulvales 275 



Sph&roplea was first described by Agardh in 1824, but the first good account of the 

 genus was by Cohn ('56). Since then others have added further and more detailed 

 information, notably Rauwenhoff ('88), Golenkin ('99) and Klebahn ('99). 



There is only one species, Spheeroplea annulina, with a rather local distribution in 

 Europe, Asia, Africa and America. It occurs in pools, but more especially on plains 

 which are liable to be flooded. Klebahn ('99) has suggested that Sph. annulina should be 

 split up into Sph. Braunii and Sph. crassisepta, but K. Meyer ('06) rightly contests this 

 view, and states that the variability is so great that only one species can be recognized, of 

 which forma Braunii and forma crassisepta are the extreme forms. 



Order 4. ULVALES. 



The order Ulvales is characterized by an expanded parenchymatous 

 thallus, attached when young at one point to the substratum by rhizoids. The 

 cells are uninucleate, with a single parietal chloroplast, which contains one 

 pyrenoid and is often sufficiently massive to fill most of the cell. In some 

 cases the expanded thallus is flat and more or less irregularly lobed, but in 

 others it is tubular and to some extent bent and contorted. 



The thallus is sometimes propagated by proliferation or gemmation. 

 Asexual reproduction takes place by zoogoiiidia, and there is a gamogenesis 

 of isoplanogametes with two cilia, the zygote germinating at once. 



The structure of the thallus indicates that the Ulvales are a rather 

 isolated group which has probably had an origin in the Tetrasporine series 

 of the Protococcales. 



Family Ulvaceae. 



The Ulvacese are the only known family of the Ulvales, and the Alga? 

 comprised in it are mostly marine and estuarine. The thallus may be a flat 

 plate, ribbon-shaped or widely expanded, or it may be tubular and intestini- 

 form. The simplest form is seen in Monostroma in which the flat thallus of 

 the adult Alga is one layer of cells in thickness. In Ulva and Letterstedtia 

 the thallus is composed of two layers of cells. 



Monostroma has both freshwater and marine representatives, and in its 

 early stages the thallus is bladder-like, splitting later to form an irregular 

 membranous expansion which often floats quite freely. The cells may be so 

 compactly arranged that their outlines are polygonal, or they may be 

 distantly disposed and rounded in contour. In the latter case they are 

 usually arranged in groups of four (Monostroma bullosa). In Enteromorpha 

 and Letterstedtia the thallus is usually branched (consult figs. 177 A and 

 178), and in the latter genus lateral 'foliar' appendages are developed which 

 subsequently fall off leaving the older parts of the thallus with irregularly 

 toothed margins. 



182 



