26 PROTOPLASM 



V)ecaiise they cannot be seen in living material. Whether or 

 not they are real or artifacts is of interest not only in so far as 

 the fibers themselves are concerned but as a critical attitude 

 toward all structures seen in killed and stained material, some 

 of which are certainly artifacts. Spindle fibers are usually to 

 be seen only in fixed material, but as they appear in practically 

 all tissue cells killed with a variety of fixatives, and as they 

 always occur in the same position relative to other cell parts, 

 they must represent something in the living cell. This may be 

 either a preexisting structure of a purely anatomical nature or a 

 field of force which brings about the linear orientation of particles, 

 such as a magnet does to iron filings. Recent work by Belaf 

 supports the reality of spindle fibers. He has followed division 

 in living cells, made constant comparison with cells that have 

 been killed, and finds similar structures, including spindle fibers, 

 in both. These fibers may be made visible in living (dividing) 

 cells if a mild acid is added; they disappear when the solution is 

 again made alkaline; and this may be done without permanent 

 injury to the cell, which continues its normal division. Such 

 behavior supports the cytologist's experience that spindle fibers 

 show up in cells killed with acid fixatives but not when killed 

 with basic ones. Strasburger said if you do not get spindle 

 fibers, add acid (acetic), and you will, and ever since cytologists 

 have been adding acid to their fixatives. The Belgian school of 

 botanists, represented by Gregoir, Jungers, and Robyns, does 

 not regard spindle fibers as real but merely as an indication of 

 an orientation of granules, that is to say, the fibers are not real in 

 themselves, but are the expression of something else which is real. 

 There are others who regard spindle fibers as actual fibers or 

 threads. 



The Vacuole. — The vacuole is one of the most important of 

 cell parts. The typical mature plant cell contains a large central 

 vacuole or cavity which is filled with water, salts, sugars, and 

 other substances (Fig. 4). The protoplasm of younger plant 

 cells is devoid of a large central vacuole and contains, instead, 

 many very small vacuoles. It is, therefore, said to be vacuolate. 

 This is probably rather generally true of protoplasm. Dangeard 

 has said that there is no cell without a vacuole. In certain 

 organisms, particularly the Protozoa, the vacuole attains a very 

 special form, with, possibly, distinct functions. 



