GROWTH AND MOVEMENT 425 
tracheae and the thin portions of the wall in the scalariform tracheids 
of ferns. In case great thickening occurs, the death of the protoplast 
is likely to follow, and this is regularly the case in tracheary tissue. 
When that occurs, further modification of the wall is possible only by 
the agency of adjacent live cells, by chemical reaction in the wall sub- 
stances, or by mere impregnation with solutes which may be precipitated 
or absorbed. So proceed such changes as the coloring and other altera- 
tions which mark the heart wood of trees. 
Tension of tissues. When growth is finally at an end in any region, 
it is found that the various tissues have not grown equally. Hence there 
exist strains or tensions; one region is compressed, another is stretched. 
These inequalities tend to adjust themselves if the regions are parted 
artificially, as when the pith, the bark, and the wood are separated from 
one another. Similarly, tensions due to unequal turgor exist (see p. 310). 
All these strains acting in different directions within the structure tend 
to increase its rigidity, just as do like strains in a latticed girder or a 
bridge truss. 
Conditions. The conditions for growth are first of all an adequate 
supply of water, for unless turgor of a meristem region is maintained, 
division of the cells is impossible, and unless an adequate amount of 
water be present, enlargement of formed cells is limited. Secondly, 
there must be a sufficient supply of constructive materials ; for though 
water plays an extraordinary part in enlargement, there is needed much 
food for making new cytoplasm as new cells arise by division and en- 
large. Nuclear material, cell-wall stuff, and much besides must be 
steadily constructed by the protoplasts, and the growing region is there- 
fore the seat of intense chemical activity. Thirdly, oxygen is necessary, 
probably to permit the metabolism in general, and especially the res- 
piratory changes, to proceed properly. For though growth has been 
observed in the absence of oxygen, it is quite limited, and, having been 
detected only by measurement, was probably due solely to the disten- 
tion by water. Cell division also is checked by lack of O 2 . Lastly, 
growth, like all other phenomena, goes on only within certain limits 
of temperature, other conditions being suitable. The optimum (dif- 
ferent for different plants and for the same plant under different con- 
ditions) usually lies between 25 and 32 C., and the extremes are near 
o and 42 C. Any one of the conditions named may likewise vary 
within rather wide limits, and any one being unfavorable may retard 
or stop growth. Yet when all the conditions are favorable, periodic 
