248 Structural and Physiological Botany, = 
_ The important points of distinction between the different 2 
forms depend either on the degree of development of the ey 
individual cells, or on the mode in which they combine. 
Even among the Unicellular Algze the greatest variety of 
forms is found, from the globular PVleurococcus (Fig. 59, 
p. 37), and the rooting spherical cell of Botrydium (Fig. 81, 
Pp. 55), to the species of Caulerpa (Fig. 380) of tropical seas, 
two feet in length, and simulating the appearance of stem, 
leaves, and roots. Sometimes the single cell branches 
copiously, its branches becoming most intimately connected 
with one another in such a peculiar way that a tissue-like 
_ structure is produced, as in Celeochete scutata, and a section 
through the whole presents the appearance of a number of 
cell-cavities, where we have actually only protuberances of 
a single cell. Sometimes these protuberances he in such 
regular layers one over another, that a transverse section 
might be imagined to exhibit one medullary and two cor- 
tical layers, as in Udotea cyathiformis. A number of Uni- 
cellular Algze often constitute a colony of cells genetically 
and. organically united, which behaves to its environment 
‘in all respects as a single individual (Figs. 58, 59, p. 37). 
The Multicellular Algze are either filiform, the cells arranged 
in a single row or thread (Fig. 54, p. 32); or the cells form 
a flat surface or single layer only one cell thick (Fig. 82, 
p. 56); or, finally, a solid mass of cells in three directions. | 
Within these three classes even a greater variety of form 
prevails than in the Unicellular Algz ; some of the larger 
species of Fucus, for example, often bearing a resemblance 
to branched trees. In the massive Alge it is very common 
for there to be a differentiation to this extent into epidermis 
and fundamental tissue, that the outermost layers consist of 
smaller cells with thicker walls, while the interior cells are often. 
much larger, and sometimes of great length. It 1s, however, 
| possible at most to speak only of a pseudo- parenchyma ; 
there is nothing of the nature of primary parenchyma or 
cambium ; and the entire oe of growth is governed by ~ 
I 
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