AN ENCYCLOPADIA OF HORTICULTURE. 
231 
Prothallus, or Prothallium—continued. 
antheridia and archegonia; and the young, leafy plant 
continues to draw nourishment from it for a time, as in 
Ferns. There is only one form of spores in this group, 
and the Prothalli are, therefore, all alike in each species. 
The second group, Selaginellew, is largely cultivated in 
greenhouses, and the cycle of development has been 
fully studied. In this group, spores of two kinds are 
produced in sporangia, in the axils of the leaves, near 
the tips of branches of the leafy plants. The two kinds 
are the microspores and the macrospores, which produce 
male and female Prothalli respectively. The microspores 
(from mikros, small, and spora, a spore) are much smaller 
than the macrospores (from makros, large, and spora). 
The Prothalli developed from both are very much re- 
duced in size, as compared with the Prothalli already 
described ; indeed, the greater part, or even the whole, 
of their development, goes on inside the spores. The 
peculiarities of development of these Prothalli have 
_ been very carefully investigated, and described in detail, 
by Millardet, and by Pfeffer. The male Prothallus 
is developed entirely in the interior of the micro- 
spore. In this, a small part (the vegetative cell) is first 
cut off, and the remaining contents are divided by cell 
walls into six or eight cells, and these (or only certain 
of them in some species) divide still further to form the 
parent cells of the antherozoids. In each of these, a long, 
slender, spiral antherozoid, with cilia at one end, is pro- 
duced. The macrospores, while still in the sporangium, in 
Selaginella, show a mass of small-celled tissue, like a cap, 
at one end, covering a very large cell, which occupies 
the greater part of the spore. After the spore has been 
for some time out of the sporangium, this large cell 
becomes filled with a mass of cells of comparatively large 
size, individually, which Pfeffer regards as analogous to 
the endosperm in the seeds of angiosperm flowering 
plants. The cap above this mass is the Prothallus, and 
this increases in size, and archegonia form in it, be- 
ginning at the apex, and gradually forming at a greater 
distance from the apex. The coats of the spore burst 
above the Prothallus, which projects a little. The struc- 
ture of the archegonium and of the oosphere, and the 
mode of fertilisation, are similar, in the important 
points, to those above described as occurring in Ferns ;: 
and so, moreover, is the development of the leafy plant.’ 
In the nearly allied genus Isetes the development is 
much like that in Selaginella, but no endosperm is 
formed in the macrospore. The Rhizocarpew agree, to a 
considerable extent, with the Selaginellew. 
The great interest of the Prothallus in Selaginellee — 
and the allied forms rests in the light the study of it 
throws on the processes of reproduction in Phanerogams 
(see Ovule and Pollen). The homologous stages, or 
what are at present regarded as such, may be briefly 
stated as follows. In Phanerogams, the pollen grains re- 
present the microspores, and the multicellular nature 
of the pollen corresponds to the multicellular microspore 
of Selaginella, with its rudimentary Prothallus repre- 
sented by the vegetative cell. The Gymnosperms and 
the Angiosperms differ as regards the ovule. In Gymno- 
sperms, the temporary endosperm is regarded as repre- 
senting the Prothallus inclosed in the embryo-sac as its 
macrospore ; the corpuscula represent archegonia, the 
rosette cells represent the neck of the archegonium, and 
the central cell of the corpusenlum represents the 
oosphere. As already stated, the large-celled tissue in 
the macrospore of Selaginella is regarded by Pfeffer and 
Sachs as representing the endosperm that develops in 
and the endosperm has the significance already stated. 
PROTOPLASM (from proton, first, and plasma, 
formed matter). A word frequently used by students of 
the microscopic structure of plants and of animals. 
The term was first proposed, in 1846, by the dis- 
tinguished German botanist, Hugo von Mohl, and is 
still used in the sense employed by him, to denote the 
transparent, soft, semi-fluid, jelly-like substance found 
in young, living cells of all plants. He was the first to 
appreciate the true importance of this substance. Before . 
his observations were made, it had been very generally 
believed that the wall which bounds each cell, and remains 
very evident after the cell contents have been emptied 
out, was the essential part of the cell; and the name 
“cell” was given to the space inclosed by the cell wall. 
It seems to have been first used, in 1665, by the English 
microscopist, Robert Hooke, who says: “Our micro- 
scope informs us that the substance of cork is altogether 
filled with air, and that that air is perfectly inclosed in 
little boxes or cells, distinct from one another.” Von 
Mohl recognised that the really essential part of the 
cell is the Protoplasm; and that by it other cell contents 
and the cell wall are produced. This view has been fully 
confirmed by later investigations, and also by the fact 
that among Ferns, Mosses, and other flowerless plants, 
the essential reproductive cells, for a time, consist of 
Protoplasm alone, without a cell wall. 
In young, growing tissues, such as the tip of the root 
of a Bean, or of any other large seedling, the Protoplasm 
at first frequently fills the space bounded by the cell 
wall. At one place lies a denser, round or oval mass, 
also composed of Protoplasm, called the “nucleus,” with a 
clearly-defined edge. As the cell grows larger, the Pro- 
toplasm does not increase so much as to fill the space 
within the cell wall. Cavities appear in it, occupied 
by fluid or cell sap. These are, at first, separated by 
plates of Protoplasm; but, with continued increase in 
size of the cell, the vacuoles unite, and form one large 
cavity in the centre, occupied by cell sap; and the Pro- 
toplasm forms only a layer lining the cell wall. ; 
When a living cell is Jaid in strong glycerine or in 
alcohol, the water of the cell sap and of the Protoplasm 
is drawn out of the cell by these fluids, and the contents 
shrink away from the cell wall, leaving an empty space 
etween them and the wall all round. The outer surface 
of the shrunken mass is clearer and less granular than 
the rest, and looks almost like a distinct coat. It was 
formerly known as the “primordial utricle,” but is now 
more usually called “ectoplasm” (from ektos, outside, 
and plasma). The inner substance, called “ endoplasm 
(from endon, inside, and plasma), is more granular, and 
incloses starch grains and other bodies connected with 
the nourishment of the tissues of the plants. 
Living Protoplasm ta eit —— e —* 
changes of composition, ing into it new , formi 
new bodies or products, and getting rid of materials 
that have done their work, and must be thrown out. 
All this implies constant changes in the position of the 
minute particles of which Protoplasm consists, though 
these movements are too slow, and the particles are 
usually too small, to permit of their being followed under 
the microscope. But in many cells (probably in most) 
the Protoplasm is seen to be moving round and round 
the cell, if it forms only a layer lining the cell wall; or 
it may be seen to move along the slender plates between 
the vacuoles, from the outer layer inwards towards the 
layer around the nucleus, and again outwards. Often 
a thin thread or plate shows two streams on its sides 
moving in reverse directions. Cells that consist of 
Protoplasm without a cell wall, are usually able to move 
freely about in water by moving fine threads or cilia, 
or by pushing out pseudopodia, or outgrowths, from the 
raiti. and flowing towards these, e.g., in some stages 
of Myzomycetes, such as Flowers of Tan, and other 
Fungi closely related to it. o 
— 
