CELLULAR STRUCTURE 37 



The group of algae to which belongs the 

 green sea-lettuce, so common on some of our 

 coasts, especially where the sea-water is con- 

 taminated by sewage effluents, furnishes 

 beautiful examples of the simpler stages in 

 the evolution of a flattened leaf-like type of 

 thallus, Ulva, the sea-lettuce in question 

 (Fig. 3), is somewhat advanced, for it consists 

 of cells which are arranged in two layers, but 

 otherwise division occurs in the cells of each 

 layer in such a way as to increase the area of 

 the surface. The multiplication of cells is not 

 very uniform over the whole surface, those 

 nearer the margins dividing and growing 

 faster than those nearer the middle line of 

 the leaf-like plant. Thus the surface of the 

 frond is thrown into folds and wrinkles as 

 the necessary consequence of this unequal 

 growth, But that there is some co-ordinating 

 influence at work amongst the cells is shown 

 by thejact that this wrinkling does not become 

 excessive, and the plants assume a fairly defi- 

 nite form which makes any given individual 

 easy to recognise as belonging to this and no 

 other species. The Ulva plants are securely 

 anchored to stones and other supports by 

 a special development of the cells near the 

 base of the plant. These grow out into long 

 filamentous strands, and adhere very closely 

 to the surface of the rock. The specimens 

 one often sees washed up after a storm are 

 usually the upper parts of the plants, which 

 have become torn off by the waves. 



