Water 



335 



In such cases as these, one is forced to conclude that much more vascu- 

 lar tissue is normally developed than is required to carry the normal 

 transpiration stream. The relation so frequently observed between 

 area of transpiring surface and cross-sectional area of conducting tissue 

 may therefore be simply another instance of developmental correlation, 

 of one tissue keeping step with another (p. 107), and may be without 

 causal significance for the differentiation of vascular tissue. 



There are other ways in which water may have morphogenetic sig- 

 nificance. Positive hydrostatic pressures often occur at the time of early 

 and rapid leaf growth, and leaves developing then tend to be large and 

 to have shallow lobes. A little later, when sap pressure is lower or absent, 

 the leaves are smaller and the lobes deeper. Experiments of Pearsall 



•OOi n n n 



75- 



PERCENT 



OF 



FLORETS 



CLEISTOGAMOUS 5Q 

 BLACK, 



CHASM06AM0US 

 WHITE 25 



PERCENT OF WATER 



Fig. 14-8. Relation between the percentage of water in the soil and the percentage 

 of cleistogamous florets in Stipa. ( From W. V. Brown. ) 



and Hanby (1926) tend to confirm this, since when they applied con- 

 siderable hydrostatic pressure to stems while the leaves were develop- 

 ing, the lobes were shallow, but they were deeper under less pressure. 

 The angle at which the lateral veins go off in palmate leaves seems to 

 affect their ability to deliver water. Where this angle is more than 90° 

 the flow of water under pressure is reduced. This fact may be related 

 to the determination of leaf shape. 



Osmotic pressure has important morphogenetic effects since it is one 

 of the factors determining the amount of cell enlargement. Various cir- 

 cumstances affect the osmotic concentration of the cell sap. One of these 

 is chromosome number, for Becker (p. 40) observed that in a polyploid 

 series of moss cells the concentration varied inversely with the degree of 

 polyploidy. Schlosser ( 1935 ) found in tomato that maternally inherited 

 differences in osmotic concentration in the cytoplasm, as well as in the 



