626 MICROSCOPIC FORMS OF VEGETABLE LIFE-THALLOPHYTES 



to nutrition ; while the principal parts of the nutritive apparatus, 

 which are at first so blended into a uniform expansion or thallus 

 that no real distinction exists between root, stem, and leaf, are 

 progressively evolved on types more and more peculiar to each 

 respectively, and have their functions more and more limited to 

 themselves alone. Hence we find a ' differentiation,' not merely in 

 the external form of organs, but also in their internal structure, its 

 degree bearing a close correspondence to the degree in which their 

 functions are respectively specialised or limited to particular actions. 

 But this takes place by very slow gradations, a change of external 

 form often showing itself before there is any decided differentiation 

 either in structure or function. Thus in the simple Ulvacece, what- 

 ever may be the extent of the thallus, every part has exactly the 

 same structure, and performs the same actions, as every other part, 

 living for and by itself alone. And though, when we pass to the 

 higher seaweeds, such as the common Fucus and Laminaria, we 

 observe a certain foreshadowing of the distinction between root, 

 stem, and leaf, this distinction is very imperfectly carried out, the 

 root-like and stem-like portions serving for little else than the 

 mechanical attachment of the leaf-like part of the plant. There is 

 not yet any departure from .the simple cellular type of structure, 

 the only modification being that the several layers of cells, where 

 many exist, are of different sizes and shapes, the texture being 

 usually closer on the exterior and looser within^ and that the tex- 

 ture of the stem and roots is denser than that of the leaf-like expan- 

 sions or fronds. The cells of the Phceosporece contain a substance 

 closely resembling starch, and an olive-brown pigment, which they 

 share with the Fucacece, known as phyco-phcein or fuco-xanthin. The 

 group of olive-green seaweeds presents us with the lowest type in 

 the family Ectocarpacece, which, notwithstanding, contains some of the 

 most elegant structures that are anywhere to be found in the group, 

 the full beauty of which can only be discerned by the microscope. 

 Such is the case, for example, with Sphacelaria, a small and delicate 

 seaweed, which is very commonly found growing upon larger algre, 

 either near low-water mark or altogether submerged, its general 

 form being remarkably characterised by a symmetry that extends also 

 to the individual branches, the ends of which, however, have a 

 decayed look. The apical cell of each branch is uncorticated, and 

 frequently develops into a hollow chamber of considerable size, 

 termed a sphacele, and filled, when young, with a dark mucilaginous 

 substance which, at a later stage, becomes watery. The Sphacela- 

 riacece are propagated in a non-sexual manner by peculiar buds or 

 gemmae known as propayules. 



The ordinary mode of propagation of the Phceosporece is by non- 

 sexual zoospores ; and these are of two kinds, produced respectively in 

 unilocular and multilocular zoosporanges. The former are compara- 

 tively large, nearly spherical, ovoid or pear-shaped cells, the contents 

 of which break up into a large number of zoospores. The multi- 

 locular zoosporanges have the appearance of jointed hairs, and are 

 divided internally into a number of chambers, each of which gives 

 birth to a single zoospore. The zoospores from the unilocular 



