PRACTICAL WORK 125 



new protoplasm can be formed, and growth necessarily 



ceases unless fresh supplies of combined nitrogen in 



a suitable form, i.e. as nitrate, can be obtained. The 



ordinary green plant cannot directly use the free gaseous 



nitrogen of the air to build up its proteins. And the 



plant cannot develop successfully, and dies sooner or 



later, in the absence of any one of the other essential 



elements. 



PRACTICAL WORK. 



(i) Soak the leaf provided in alcohol and note that a green 

 pigment is gradually extracted from the leaf. After an hour's 

 soaking pour some of the green solution into a test tube and 

 examine with the hand spectroscope. Compare with (6) . 



(2) Strip off part of the colourless surface layer of ceUs (epi- 

 dermis) of the leaf of Gladiolus provided. Below you will see 

 colourless ribs (veins) with strips of green tissue (mesophyll) 

 between. Cut out a small portion of this green tissue with the 

 point of a sharp scalpel or penknife and mount in a drop of 

 water, taking care to wet the tissue thoroughly. Put on a cover- 

 slip and examine under the low power. Note the small cells with 

 green dots, and the air spaces (now partly filled with water) between 

 them. Examine with the high power and carefully draw one 

 of the cells of wl^ich you can get a clear view, marking the cell 

 wall, the cytoplasm lining the wall with chloroplasts embedded in 

 it, the central vacuole, and the small shining spherical nucleus. 

 Stain a portion of the green tissue with haematoxylin for ten 

 minutes and note that the nucleus especially absorbs and becomes 

 coloured by the stain. 



(3) Examine under the microscope in a drop of water a whole 

 leaf of the water weed Elodea. This is a " water leaf," wholly 

 submerged in nature and of very simple structure, consisting, 

 except for the veins, of only two layers of cells, all containing 

 chloroplasts. The structure of each cell is essentially the same 

 as that of the mesophyll cell of Gladiolus, though the cells are 

 much larger and oblong in shape . The nucleus is generally dififtcult 

 to detect : it is often hidden in a clump of chloroplasts. When 

 visible it is seen as a rather large, pale, often faintly granular oval 

 body. Note that all the chloroplasts are embedded in cytoplasm : 

 none of them lie in the large central vacuole. The cytoplasm 

 may sometimes be seen streaming actively round the cells, carrying 

 the chloroplasts with it. (Cf. p. 85.) 



