54 Structure of Hydroclathrus. 



The form of Hydroclatlims sinuosus is that of a hollow sphere, with a 

 slightly corrugated surface, the largest specimens examined having a 

 diameter varying from two and a half to three inches. The plant adheres 

 to stones and to other Algae, but has no special organ of attachment. At 

 first it probably consists of a solid mass of cells, but this condition is only 

 transitory, since the youngest plants examined have always been found to 

 be hollow, while the cells of the layer which borders the hollow interior 

 generally present a ragged appearance, as if they had been torn apart from 

 each other. 



A section of the thallus (plate XIV., fig. 2) shows that it is made up of 

 from five to six layers of cells ; those forming the outermost or epidermal 

 layer are small closely packed polygonal cells, each with a well-developed 

 nucleus and dense protoplasmic contents. Below them come one or two 

 layers of cells larger than those of the epidermis and with thinner walls. 

 The innermost cells of all resemble those above them, excepting that they 

 show a marked increase in size, and take on a more spherical form owing 

 to the absence of pressure. When they have reached their limit of growth 

 the walls become torn, either on account of the constant tension exerted 

 by the growing thallus or by degeneration of the substance of their cell- 

 walls. 



The peripheral walls of the epidermal cells increase slightly in thick- 

 ness, thus forming a kind of cuticle which covers the whole plant, and is 

 ultimately pushed off as a result of the outward growth of the epidermal 

 cells below it during the formation of sporangia (plate XIV., fig. 3). 



H. sinuosus has neither an apical cell nor any area of special growth, 

 and increase in size takes place by division of the epidermal cells. 



While the plant is still quite young, there is no indication of the 

 formation either of cryptostomata or sporangia, and the epidermal layer 

 has everywhere an uniform appearance ; but, as the thallus increases, a 

 surface view of the epidermis (plate XV., fig. i) shows that localised changes 

 are taking place simultaneously; an isolated cell or several cells in a group 

 become separated off from the surrounding epidermis, each loses its 

 polygonal shape and becomes cylindrical. This change in appearance 

 denotes the first stage in the formation of a cryptostoma. In a radial 

 section of such a group each cell is seen to be divided by a transverse 

 wall, but there is no indication of such longitudinal division as occurs in 

 neighbouring epidermal cells. The lower of the two cells again divides 

 transversely, and this method of division continues till a long row of 

 cells has been formed, making in fact a hair. Simultaneously with the 

 formation of these hairs, the cells immediately surrounding them undergo 

 similar changes, and thus the cryptostoma enlarges radially. Meanwhile 



