74 Practical Plant Biology. 



of water is inimical to the production of conjugation-tubes sup- 

 ports this view. 



PRACTICAL WORK. 



Mount a few filaments of Spirogyra and cover. 



First examine and sketch with low power. Estimate the dimensions of 

 the cells by means of the Ghost-micrometer. 



Examine with the high power. Notice the course of the spiral chloro- 

 plasts by use of the fine adjustment. Identify the cell-wall and focus 

 particularly to observe its thickness. By closing the diaphragm observe the 

 layer of cytoplasm in which the chloroplast is embedded. Look for motion 

 in its outer layer. Observe the serrated margin of the chloroplast, its mid-rib 

 and pyrenoids. Focus for the nucleus and observe its membrane and 

 nucleolus. Notice the capsule of cytoplasm covering over its surface and the 

 filaments extending from it to the layer of cytoplasm lining the wall. Trace 

 these filaments by means of focussing from the capsule to their outer ex- 

 tremities. 



Make three drawings of the cell as seen at three different levels : (i) Focus 

 for an optical plane or section cutting the cell just within the near curved 

 surface of the cell- wall and tangential to the nearer turns of the chloroplast. 

 Fill in all the details in your sketch which you can observe without changing 

 the focus. This upper tangential section should show a continuous sheet of 

 cytoplasm lying inside the part of the cell-wall nearest the observer and in 

 this sheet those portions of the chloroplast which are approximately in the 

 same plane. These appear as oblique straps in the field, their ends passing 

 out of focus. As you focus downwards (2) the median optical section comes 

 into view, and the near parts become hazy and disappear. The cell-wall 

 appears as a very narrow refringent band outlining the whole cell. The cyto- 

 plasm is within this and is much less plainly delimited. The chloroplast 

 appears as a number of isolated crescentic green patches in the cytoplasm. 

 The hollows of the crescents are turned towards the cell-wall. Centrally 

 may be seen the nucleus its membrane and nucleolus. Usually a tuft of 

 cytoplasmic filaments takes origin from the cytoplasmic capsule of the nucleus, 

 where the latter is closest to the peripheral cytoplasm. Focus still deeper 

 and you will see (3) the further tangential optical section. It resembles in 

 all respects the nearer one, save that the slope of the portions of the chloro- 

 plasts in focus is in the opposite direction. 



From the consideration of these three sketches reconstruct a transverse, 

 section of the cell passing through the nucleus. . 



