38 FUNDAMENTALS OF CYTOLOGY 



division throughout the Hfe cycle and that with the accumulation of cell 

 sap in the tonoplasts these extended to form the membranes of vacuoles. 

 The vacuole membrane is still called the iono-plasi. It is now more gen- 

 erally believed that vacuoles arise bj^ a hydration and an accumulation 

 of certain cytoplasmic colloids, the membrane being formed by the cyto- 

 plasm at the interface. It is being found very difficult, however, to 

 demonstrate that vacuoles do arise anew in such a manner. 



The tonoplast resembles the plasma membrane in physical consistency 

 and to some degree in its semipermeability. It appears, however, to be 

 more resistant to injur}' and may manifest semipermeabilit}'^ after the cell 

 is killed. Its permeability to certain substances may be remarkably low, 

 the concentration of acids in the vacuoles of some cells being greater than 

 enough to kill the cell if they were applied to it externally. This is 

 notably true of food vacuoles of protozoa during digestion. The results 

 of permeability studies are thought to indicate that the tonoplast contains 

 more lipide than the plasma membrane. 



The cell sap in plant vacuoles of the common type is a slightly viscous 

 fluid composed of water and substances of many kinds in molecular or 

 colloidal solution. Salts, sugars, organic acids (oxalic, malic, citric, etc.), 

 glucosides, alkaloids, amides, proteins, enzymes, tannins, and other 

 compounds can be identified in different cases. Some cells secrete visible 

 globules of protein, gums, resins, and other materials in such amounts that 

 the sap has a milky appearance. In extreme cases such cells form exten- 

 sive systems of latex tubes ramifying through the other tissues of the plant ; 

 familiar examples are the dandelion and the rubber tree. Globules of 

 rubber are elaborated in the cytoplasm and secreted into the sap in 

 Ficus. Crystals of various compositions also may occur in the cell sap. 



The reactions of cell sap to chemical tests and in staining procedures 

 may vary widely in different plants and at different stages of development 

 in a given tissue. This is due both to differences in pH and to the pres- 

 ence or absence of certain particular compounds. In meristematic tissues 

 the cell sap ordinarilj'- has a slightly alkaline or a neutral (pH 7) reaction, 

 but as the vacuoles enlarge in the differentiating cells it soon becomes 

 decidedly acid, the pH falling to 5 or even lower. Phenolic compounds 

 such as tannin also have a profound effect upon stainabilit.y, the same cell 

 sometimes showing vacuoles of two colors correlated with differences in 

 tannin content. Of several vital stains commonly used for vacuoles, the 

 best is neutral red. It quickly accumulates in the vacuoles of living cells, 

 leaving the cytoplasm and nucleus colorless, and it remains there until 

 death occurs. 



Many vacuoles contain natural pigments in their sap. The most 

 prevalent of these are the anthocyanins, which are commonly reddish in 

 very acid saps and blue or purple in more alkaline mediums. Such 



