miscellaneous: stimuli underlying tumor-formation 513 



mitosis and to form cell-walls. The extent of the cell-enlarge- 

 ment will then depend on the amount of water imbibed and that 

 in turn will depend on the acidity of the cell sap and on the 

 corresponding extent to which the physiological control of imbi- 

 bition, exerted by the hyaloplasm, is upset. But cell division 

 will be rapid if the paralysis involves only the hyaloplasm. 



Let us take the simplest case first, that of hyperplasias 

 developed under obturated lenticels (Figs. 385 to 389). Here we 

 may suppose that some air still enters and that some vapor of 

 water and gas still escapes, but the gas-exchange is demon- 

 strably reduced to a very small fraction of what it was, that is, 

 vapor of water and carbon dioxid cannot now escape through 

 these openings as before, and air cannot now enter freely. In 

 other words, there is a stasis in the tissues under these openings, 

 less entrance of air and less movement of aerated water, with 

 more or less oxygen-hunger and with increase in cell-acidity 

 (due to products of incomplete oxidation) ; also, owing to root- 

 absorption, with increase of turgor pressure and, corresponding 

 to these changes, a hyperplasia develops. Subjected to these 

 conditions, many plants develop small tumors under the lenti- 

 cels. The character of the hyperplasia, whether few-celled or 

 many-celled, slow-growing or active, will depend on the nature 

 of the tissues, and on the extent to which the lenticels are closed 

 and the gas-exchange is interfered with. If there is less and 

 less gas-exchange, the acid condition and the oxygen-hunger 

 will be proportionately increased and the hyperplasia will be 

 very small-celled and active. If there is still considerable en- 

 trance of air and exit of aerated water and of gas either through 

 imperfect closure of the lenticels or directly through the surface 

 of the stem, the hyperplasia will be large-celled and slow-grow- 

 ing, and this seems to correspond to the facts observed. 



Every growing cell is in constant need of oxygen — must 

 have it at once or die. This is absorbed, it is now believed, 

 through its whole periphery, either directly from the air or in- 

 directly out of the aerated fluid which bathes its surface. If the 



Painted with petrolatum March 18, 1918. Photographed from a free-hand, 

 ur^stained section in water, March 26. Upper part of prohferation torn away 

 in making the section. 16 mm., 4 oc, bellows at 45. 



