Various Physical Factors 359 



tissues also changes. Schiiepp (1917) found that there was tension in the 

 growing point but that here it was opposite in its distribution from that 

 in tissue which was extending. Schneider (1926) found no tension in the 

 growing point itself but saw it first in the leaf primordia, which for this 

 reason tend to bend inward. 



What bearing tissue tension may have on differentiation or on the de- 

 velopment of form is not clear but it may be of some importance. In 

 young ovaries, however, where presumably form and structure are being 

 determined, there seems to be little tension though this increases in 

 later stages of development. A remarkable instance of tension in dead, 

 dry wood has been reported by M. R. Jacobs ( 1945), who found that if a 

 board which includes the whole width of the log is sawed at one end part 

 way down by a series of parallel longitudinal cuts the strips thus sepa- 

 rated tend to spread apart fanwise, indicating the existence of a very 

 considerable degree of tension between the inner and the outer parts of 

 the log. 



Absolute Size. Another morphogenetic factor, which in a sense may 

 be regarded as physical, is absolute size itself. It is clear, as Galileo long 

 ago pointed out in his principle of similitude, that as any body increases 

 in size its volume enlarges as the cube of the diameter but its surface 

 only as the square. Thus the ratio of surface to volume will progressively 

 decrease. In a living organism, where physiological activity is often 

 limited by the amount of available surface for interchange of material 

 between organism and environment, or between one tissue and another, 

 the surface-volume relationship is of much importance and is obviously 

 related to changes in shape and structure. 



This shows most simply in the increased elaboration of bodily form 

 as size increases. Among algae, for example, the smaller types are 

 relatively simple and compact but the larger ones, through branching 

 or surface convolution, have a much more elaborate conformation, with 

 the result that the ratio of surface to volume is not very different in the 

 two. A good example of the same thing is the difference between the 

 small, rounded chloroplasts of higher plants and the very much larger 

 chromatophores of some of the lower ones, which are elaborately branched 

 and dissected. Internal anatomy displays the same tendency, for in the 

 sporeling of a fern the vascular cylinder is a solid rod but as the plant 

 grows this soon opens out to form a hollow tube. It may later be broken 

 up into a ring of bundles or even a series of concentric rings or tubes. 

 The radial thickness of each strand thus tends to be approximately the 

 same, with the result that the surface of contact between xylem and 

 phloem, so important in the physiology of the plant, remains relatively 

 constant. Every unit mass of phloem tissue has essentially the same 

 "frontier" on the xylem as every other one, and none is limited in its water 



