PLATE VL-SPIROGYRA, DESMIDS, and DIATOMS. 



(Spirogyra chitjlir after Sachs; DesmiU and Diatom after Oersted.) 



Spirogyra (Gr. guros, a ring) is readily recognised under the microscope from the spiral bands of green-coloured protoplasm. It 

 floats in bright green masses near the surface of clear, fresh waters, such as ponds, and slips through the fingers on attempting to 

 handle it. 



The bands of coloured protoplasm are variable in their number and arrangement. They contain numerous starch-granules and oil- 

 globules, and a nucleus is present in each cell. This condensed portion of the protoplasm is surrounded by a layer of protoplasm, 

 which sends delicate threads towards the cell-wall, giving the nucleus a star-like appearance. There is also a layer of protoplasm lining 

 the cell-wall, to which these threads are attached, and this lining is made very evident by the application of iodine, which causes the 

 protoplasm to contract and withdraw itself from the wall. The protoplasm is broken up into shreds and bands, because, being unable 

 to fill the cell, the cavities are filled with cell-sap, and these spreading and increasing finally leave the protoplasm in this scattered form. 

 Protoplasm thus on the stretch, as it were, displays much of its intimate nature, which is concealed in the more uniform condition. 



Multiplication of the cells takes place by Division, and Reproduction by Conjugation. 



Fig. 1. Either take a small portion of the water in which odd pieces are floating, or a minute portion of the green mass, and 

 examine under highest power. 



Long filaments made up of cells, with distinct walls and green spiral bands, in which numerous granules are 

 visible. 



Diagram. — Showing arrangement of bands. 



Careful focussing is necessary to make out the exact continuity of the bands, and this may be made out better after 

 treatment with reagents than in natural specimens. 



In this particular species the bands are arranged in two spirals, which intersect each other. In S. longata (Fig. 4) 

 there is but a single spiral band. > 



Fig. 2. Stain with Iodine. 



Iodine makes the nucleus prominent, turns the starch-granules blue, and causes the layer of protoplasm lining 

 the cell-wall to contract about the spiral bands. This layer of protoplasm has received the name of "primordial 

 utricle," but it is simply a portion of the protoplasm which lines the cell-wall. 

 Fig. 3. As division takes place during night, in order to get cells in the act of division place them in alcohol shortly after mid- 

 night and examine with highest power. 



The cellulose is seen to be extending inwards on each side. 



Fig. 4tf. Cell in the living state, with single nucleus and regularly-arranged bands. 



b. Protoplasm contracted by the alcohol. Infolding of the protoplasm lining the wall, and cellulose formed in the notch. 



Two nuclei formed during division, one for each new cell. 



c. Infolding further advanced, which would ultimately form a complete partition across. 



FigS. 5 and 6. Conjugation. 



Two filaments lay themselves alongside each other, and adjoining cells of each filament throw out pockets 

 simultaneously towards each other, which eventually meet and form a connecting tube between the two cells. The 

 contents of one cell pass over and fuse with that of the other, the nuclei also coalescing, thus producing a Zygospore, 

 as in Fig. 6. 



Fig. 7. Germination. 



The outer wall of the Zygospore ruptures, and the innermost layer protrudes as a filament, gradually growing 

 and forming transverse partitions until a proper filament is produced. 

 Desmids (Gr. desmos, a band) are beautiful, minute, green plants, found in fresh water, and consisting usually of a single cell. 

 The cells are generally divided into two symmetrical halves, and the coloured protoplasm is arranged in bands. 

 Multiplication by Division is shown in next Plate. Sexual reproduction by Conjugation is shown here. 



Fig. 8. Different views of Cosmarium, showing the two halves and the coloured bands. 

 Fig. 9. Two cells approach one another, the narrow waist ruptures, and the contents of each fuse. 

 Fig. 10. A single rounded mass is formed, with the empty halves of each Desmid still adhering to it. 

 Fig. 11. The Zygospore secretes a cellulose wall, which grows out into beautiful spines. 

 Fig. 12. The Zygospore escapes from its case and begins to germinate. 

 Fig. 13. Zygospore divides into two new Desmids, which lie across each other. 

 Diatoms (Gr. dia, through ; temno, I cut) are so named from the common genus Diatoma, in which the cell-walls, or Frustules (Lat. 

 frustum, a fragment), remain connected in a zigzag fashion after each division, looking like a continuous structure cut up into a number 

 of similar fragments. Various forms are sure to be met with while examining fresh-water Algae, Euglena, and the like. 



They are unicellular like the Desmids, but are yellowish in colour, have not the characteristic median constriction, and their cell-walls 

 are silicious, exhibiting on their surface those beautiful markings which are a never-ending source of delight and interest to the micro- 

 scopist It is owing to this indestructible character of the cell-wall that Diatoms form geological deposits, and their beautiful structure 

 has been preserved as finely as those living at the present day. The Diatom muds, of a pale straw colour, beneath peat-mosses, have 

 acquired great importance recently from being used in the manufacture of dynamite, which is a combination of the silicious material with 

 nitro-glycerine. 



They exhibit slow movement from place to place, and exposed to light in considerable numbers they evolve oxygen. 

 Multiplication takes place by Division, Reproduction by Conjugation. 



Fig. 14. Diatoma, a very common form. The cells formed by successive divisions remain slightly attached. 



Fig. 15. Conjugation of Frustulia saxonica. 



(a.) Two Diatoms beside each other surround themselves with a gelatinous mass, the valves then fall apart like an 



opened book, and the contents of each come together, but do not mix. 

 (b.) Next, the two contents clothe themselves with a delicate membrane, elongate, and form two Zygospores. 

 (c.) Each Zygospore now forms two valves, and becomes fully formed. 



