2 LIVING CELLS (1) 



Exercise I 



forceps place a cover glass over it, putting one 

 edge of the cover glass down first and then 

 letting the other end down slowly so as to drive 

 out all bubbles of air. Examine under the low 

 power of the microscope. The epidermal cells 

 of the onion are typical plant cells in that they 

 consist of (1) a cellulose cell wall, (2) a thin 

 layer of cytoplasm which lines the cell wall, 

 (3) a nucleus, and (4) a large central vacuole. 

 Observe as many features of the living cell as 

 you can. Stain a piece of this tissue with aceto- 

 carmine. This is a dye which stains basic pro- 

 teins red, made up in 45% acetic acid which 

 coagulates protoplasm (like cooking an egg). 

 Sketch one cell, showing cell wall, cytoplasm, 

 vacuole, and nucleus. 



Human epidermis 



Having seen some onion skin under a micro- 

 scope, you may enjoy seeing your own. With a 

 reasonably clean fingernail, or the blunt end of 

 your scalpel, scrape the inside of your cheek 

 lightly. Stir the scrapings into a drop of tap 

 water on a slide, cover with a cover slip, and 

 find the cells under low power. They will 

 appear as small masses of colorless, granular 

 material. Under high power, study their struc- 

 ture. This is a large, flat type of cell (squamous 

 epithelium) that, as in the onion, forms tissue 

 surfaces. Individual cells are best seen at the 

 margins of a group. Note the cell membrane, 

 junctures with neighboring cells, the granular 

 cytoplasm, and the small, rounded, highly re- 

 fractile nucleus, itself surrounded by a mem- 

 brane. Sketch one cell. Compare the cell mem- 

 brane with that of the onion cell; this is one of 

 the features by which one distinguishes plant 

 and animal cells. 



Highly specialized cells: the Elodea leaf 



Elodea is a flowering plant that grows in 

 fresh water. Pluck a young leaf and mount it 

 whole in tap water, top side up, under a cover 

 slip. Under low power find a group of elongated 

 cells near the midrib and toward the base of the 



leaf. The green structures are chloroplasts. 

 They are the organs of photosynthesis, and the 

 green chlorophyll and other pigments they con- 

 tain absorb the light used in this process. In 

 some cells you can observe a circulation of the 

 protoplasm (cyclosis). If you find this, try the 

 effect on it of changing the brightness of the 

 light. Make a sketch of a single cell, showing 

 the relation to its neighbors, and as much of 

 the internal structure as you have seen. 



You have already seen an example of the 

 cellulose wall that typically encloses a plant cell, 

 and of the delicate surface membrane that sur- 

 rounds the cytoplasm of all living cells, the 

 so-called plasma membrane. We shall take this 

 opportunity to demonstrate the relationship be- 

 tween these structures in Elodea cells. When 

 such a plant cell is laid in a salt solution more 

 concentrated than its own cytoplasm, the salt 

 solution draws water out of the cell, causing it 

 to shrink away from the rigid cell wall, so ex- 

 posing the plasma membrane. 



Mount a whole Elodea leaf as above, and 

 when you have a good field of cells in focus 

 under the microscope, replace the tap water by 

 a concentrated solution of sodium chloride 

 (2 M). This is done by using a medicine dropper 

 to place a single drop of the salt solution on the 

 slide beside the cover slip, just making contact 

 with its edge. Then touch the margin of liquid 

 at the opposite side of the cover slip with a 

 small piece of lens paper, so that the lens paper 

 draws up the liquid, sucking the salt solution 

 under the cover slip to replace the liquid you 

 have withdrawn. When you have done this 

 two or three times, the liquid under the cover 

 slip will have been completely replaced by the 

 salt solution. This is the general method used 

 for changing solutions under a cover slip, so 

 that their effects can be observed as the change 

 progresses. 



Potato 



Cut a thin slice from a freshly cut surface of 

 a potato tuber. Lay the section in a drop of 

 water on a slide and examine under low power. 



