DEPARTMENT OF EXPERIMENTAL EVOLUTION. 



gi'eater the number of ovules and of seeds per pod and the greater the 

 average weight of the seeds. 



RELATION BETWEEN SYMMETRY AND FECUNDITY. 



The relation between symmetry in distribution of ovules in the bean- 

 pod and the average number of seeds formed has been studied by Dr. 

 Harris in navy beans. It appears that in pods with unsymmetrically 

 placed ovules fewer of them, on the average, form seeds than in pods 

 with symmetrically placed (even) ovules. 



OTHER INVESTIGATIONS. 

 VEGETABLE SAPS. 



By means of the method of measuring osmotic pressures of vegeta- 

 ble saps elaborated by Gortner and Harris, Dr. Harris and Mr. John V. 

 Lawrence have made an interesting series of comparative investiga- 

 tions into the concentration of saps. 



First, they have studied the relation between the osmotic concen- 

 tration of leaf sap and height of leaf insertion in trees and have found 

 that the concentration (as determined by the freezing-point-lowering 

 method) increases from lower to higher levels, and it seems probable 

 that it is by virtue of this increased concentration at higher levels that 

 sap rises to the higher levels. 



Second, they have considered the relation of concentration of sap of 

 the whole plant to environmental conditions. Thus they find that 

 about the Desert Laboratory at Tucson the plants of the arroyo or 

 sandy wash show the lowest osmotic values, sometimes only half of 

 those of related species growing in other habitats. Next higher in sap 

 concentration came the plants of Pima Canyon, then those of the rocky 

 slopes; next those of the mesa-like slopes, and, highest of all, those of 

 the salt spots, where the osmotic values are 12 to 71 per cent greater 

 than those of plants from the other habitats studied. It appears, also, 

 that the sap concentration diminishes in the successive terms of the 

 series — trees and shrubs, half-shrubs, perennial herbs, and winter 

 annuals. In the Jamaican coastal deserts a similar high concentration 

 of cell-sap is found. In sharp contrast with the condition in the des- 

 erts, the vegetation of the rain-forest of Jamaica is characterized by low 

 sap-pressure, the concentration of the fluids of the latter being less than 

 half that of the former. 



Again, they have shown thatphanogamic plants (Loranthacese) , para- 

 sitic upon other plants, have, in general, a higher osmotic sap-pressure 

 than their hosts. It is apparently by virtue of such higher concentra- 

 tion that they are able to steal fluids, by osmosis, from their hosts. 



Dr. Harris, working with Mr. Wilson Popenoe, has been able to show 

 that of three types of the tropical fruit avocado, whose cultivation is 

 now being conducted in Florida, the Mexican type has a more concen- 



