ALGrJE. 



437 



their colour is green, an important exception to this being formed by the 

 Diatumacece, which have another special peculiarity in the existence of a 

 siliceous deposit in their walls, which remains as an indestructible skeleton 

 after the decay of the organic matter of the plants. Some of these lower 

 Algaa are found of red colour ; but in many cases, at least, this colour is 

 only characteristic of certain stages of growth of kinds which are green 

 when vegetating actively. It is very probable that most of these so-called 

 unicellular or pseudo-unicellular Algae are really not independent organ- 

 isms, but stages of growth of some other plant, perhaps of much higher 

 structure. Thus there is reason to think that these unicellular bodies 

 may not only be stages in the development of Lichens, but even of Mosses. 

 One remarkable point in their history is the length of time they persist 

 unchanged. 



A step forward in complexity of organization is made in the filamentous 

 Algae, composed of cylindrical cells attached end to end, and thus forming 

 long jointed tubes, either simple or more or less branched (fig. 512, C, 

 p. 451) : the Confervoideae and their allies, the " silk-weeds of fresh- 

 Fig. 504. 



Palmetto, cruenta: a, mass of jelly with" single cells, some dividing into two ; ft, detached 

 cells and granules; c, cells treated with sulphuric acid and iodine, showing the cellular and 

 granular contents. 



water pools, afford familiar examples of this structure ; these grow at the 

 extremity of the filaments, or interstitially by all the joints elongating 

 simultaneously. 



The Ulvece have a thallus where growth in breadth is added to that in 

 length ; some of them also acquire a certain thickness : in this way they 

 become leaf-like expansions, of membranous texture. They grow by ad- 

 ditions all round the margins of the anterior part, often lobed or divided, 

 but of homogeneous tissue throughout : the form and dimensions of the 

 thallus become more or less definite here ; the colour is mostly green, as 

 in the Oonfervoide. 



The Red Seaweeds or Rhodospermeae exhibit almost every possible 

 form between that of the branched filamentous thallus and that of a highly 

 compound or dissected leaf (fig. 510, p. 447) or a shrub-like collection of firm 

 branches ; and, moreover, the texture of the thallus varies from a simply 

 membranous to a cartilaginous or even horny substance, caused by greater 

 development of the cellular tissue, which in the higher kinds exhibits a 

 distinction between the cortical or epidermal layer and the internal spongy 

 parenchyma. The Corallines, which belong to this group, acquire a stony 

 character from the deposition of carbonate of lime in their cellular tissue. 



