AN ENCYCLOPEDIA 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 

 Perns. There is only one form of spores in this group, 

 and the Prothalli are, therefore, all alike in each species. 

 The second group, Selaginellece, 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 mnch 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 arigiosperm 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 Prothallns, 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 Iscetes the development is 

 much like that in Selaginella, but no endosperm is 

 formed in the macrospore. The Rhizocarpeae agree, to a 

 considerable extent, with the Selaginellece. 



The great interest of the Prothallus in Selaginellea: 

 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 multicellnlar 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 corpusculum 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 

 seeds after fertilisation. In Angiosperms, the embryo-sac 

 represents the macrospore, the antipodal cells, perhaps, 

 correspond to the Prothallus, the embryonal vesicle to the 

 oosphere, the helper-cells to the neck of the archegonium, 

 and the endosperm has the significance already stated. 



PROTOGYNOTJS. See Proterogyuous. 



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 laid 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 

 between 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 is constantly undergoing rapid 

 changes of composition, taking into it new food, forming 

 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 vacnoles, 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 psenSopodia, or outgrowths, from the 

 surface, and flowing towards these, e.g., in some stages 

 of Myxomycetes, such as Flowers of Tan, and other 

 Fungi closely related to it. 



