6 BOTANT 



(g) For very careful study the following method of preparation 

 should he followed : Place a fresh root-tip of Indian corn, onion, or 

 hyacinth in a 1-per-cent aqueous solution of chromic acid for twenty 

 or twenty-four hours ; thoroughly wash it for some hours in running 

 water ; place it successively in 20-, 30-, 50 , 75-, 95-per-cent, and ahso- 

 lute alcohol, allowing it to remain in each for a few hours ; then 

 transfer it to turpentine, a few hours later to a warm mixture of tur- 

 pentine and paraffin, and still later (3 to 4 hours) to melted paraffin, 

 where it must be kept for 24 to 48 hours at a temperature of about 

 60° C. When cooled the specimen will be firmly imbedded in the 

 paraffin, and may be cut into very thin sections on any microtome. 

 The sections may then be attached to a glass slip by a film of collo- 

 dion, the paraffin removed by heat, turpentine, and alcohol, and after- 

 wards stained by hsematoxylon, carmine, or safranin. The specimens 

 must now be again dehydrated by the application of 50-, 75-, 95-per- 

 cent, and absolute alcohol, the alcohol washed ofE by turpentine, Can- 

 ada balsam added, and the cover-glass put in place. When dry and 

 hard the specimen is ready for study under a very high power (1000 

 diameters or more) of the microscope. 



7. The Plant-cell. — In all common plants the proto- 

 plasm is usually found in minute masses (consisting of the 

 cytoplasm, nucleus, chromatophores, and centrospheres) of 

 definite shapes, each one enclosed in a little box (Fig. 1, w). 

 The substance of these boxes was made by the protoplasm, 

 somewhat as the snail makes its shell. Each mass of pro- 

 toplasm with its box is called a Plant-cell, and the sides of 

 the box are called the walls of the cell, or the cell-wall. 



8, The young cell-wall consists of cellulose, which is 

 composed of carbon, hydrogen, and oxygen (C,Hj„OJ. At 

 first it is very thin, but as the protoplasm grows older it 

 thickens its wall by continually adding new material to it, ^ 

 so tliat at last it may be many times as thick as at the be- 

 ginning. Moreover as it grows older other substances are 

 deposited or developed in the wall, so that it is no longer 

 pure cellulose. Thus the walls of cork and epidermal cells 

 contain cutin (suberin), those of wood-cells lignin, while 



