EXPERIMENTAL MATERIALS AND METHODS 17 



that tissues capable of rapid salt absorption after washing are those 

 which are able to synthesize protein and resume growth by callus 

 formation at a cut surface. 



In order to take full advantage of the homogeneity of the cells 

 in storage tissues, the material is used in the form of thin disks, 

 preferably not exceeding 1 mm in thickness. This minimizes any 

 differences in absorption by cells at the surface and inside the shce, 

 so that the experimental observations can be assumed to reflect 

 the uniform behaviour of individual cells in the tissue as a whole. 



Against the use of storage tissues, it has been argued that dormant 

 organs consist of highly specialized cells, which do not normally 

 regain a capacity to absorb salt once growth has stopped. The 

 process of salt uptake induced in such cells may, therefore, bear 

 little relation to that occurring in the roots of intact plants. There is, 

 however, no evidence that this is the case, and it seems probable 

 that a greater knowledge of the mechanism operative in tissue slices 

 may assist in the elucidation of salt absorption in more complex 

 systems (but see p. 13). 



4. Callus Tissue Cultures 



Although excised roots and storage tissue slices have an inherent 

 capacity for growth,* this is not always manifested during an 

 absorption experiment. With these materials, therefore, salt uptake 

 can be studied without the complicating influence of concomitant 

 growth. A study of salt absorption in non-growing tissues is, 

 however, of limited interest, and should be supplemented with 

 observations on growing cells, since growth has important effects on 

 the absorption mechanism. Ideally, homogeneous populations of 

 either actively-dividing or -expanding cells must be employed if 

 experimental data are to be adequately interpreted at the cell level. 

 As far as higher plants are concerned, an approach to these can be 

 obtained through the technique of tissue culture. 



Early methods of culturing tissues on the surface of agar proved 

 unsuitable for most physiological studies because of the slow and 

 variable growth induced. The discovery that uniform, rapidly 

 growing cultures of carrot, and other storage tissue cells in liquid 

 media can be obtained under the stimulus of growth-promoting 



* Growth is defined in this context as increase in cell number, an irreversible 

 increase in cell size, or net synthesis of protein. 



