PHILIP R. WHITE 249 



pattern can, in many cases, be radically changed. The degree and nature 

 of the change depend on the concentration and activity of the growth 

 substance used, as well as on the tissue involved. At low concentrations 

 the tendency to initial hyperhydric enlargement is only sHghtly accen- 

 tuated. The activity of the cambium, however, is greatly enhanced. 

 Instead of being relatively superficial, the cambium tends to develop 

 at a much greater depth in the fragment and to involve not a single 

 layer but a considerable region. Instead of only a few superficial cork 

 cells and deeper lignified masses, there is a prohferating region which is 

 continuous both tangentially and radially, the new-formed cells tend 

 to remain for a considerable time thin-walled, and there is formed an 

 erratic but nevertheless clearly defined phloem on one side and xylem 

 on the other. The initial arrangement of these layers may vary with 

 the region from which the tissues were excised, but by gradual re- 

 arrangement the phloem always comes finally to occupy the superficial 

 position. In addition the enlarged outer hyperhydric cells frequently 

 undergo disoriented divisions resulting in the formation of nodular 

 masses which, when they become sufficiently large, develop their own 

 peripheral cambium and proceed as distinct centers of growth. Isolated 

 areas of meristematic activity also often arise deep within the mass, 

 especially in the neighborhood of necrotic areas. These localized centers, 

 either superficial or deep-seated, give rise to the tuberculate and often 

 loose and friable character of many rapidly growing plant tissue cultures. 

 Subcultures can be made easily from such masses without important 

 trauma. Indeed new growths frequently arise where a few cells have 

 been accidentally dropped in manipulating the larger masses. 



Two facts about these cultures appear at this stage of our ignorance 

 to be of special importance. First, these cultures are still relatively 

 disorganized, the only evidence of polarity being the radial polarity 

 exemplified in the xylem-cambium-phloem orientation. In the second 

 place the whole development of these cultures and the contrast between 

 the static surviving masses discussed first and these rapidly growing, 

 highly active disorganized masses has resulted from and is dependent 

 on the presence of a certain amount of auxin in the substratum. Just 

 how this auxin functions, aside from retarding differentiation (depo- 

 sition of cell wall material) and enhancing mitosis (deposition of 

 cytoplasmic proteins), is not at present clear even in a general way. 



If the concentration of growth substances is increased somewhat more 



