REPRODUCTION AND DISPERSAL 895 



parasites cause flower primordia to develop into vegetative organs, as in the golden- 

 rod (fig. 1097). 



The influence of external factors upon sex determination. The great 

 majority of plants appear to be strictly monoclinous or diclinous, the 

 latter being for the most part strictly monoecious or strictly dioecious. 

 As previously noted, however, there are a number of species, which vary 

 between monocliny and dicliny or between monoecism and dioecism. 

 It is particularly among such plants that experimentation has been car- 

 ried on regarding sex determination. When maize (Zea Mays} is grown 

 under favorable vegetative conditions, the plants commonly are monoe- 

 cious, but when it is grown in dry, sterile soil or is exposed to weak light, 

 a small unbranched plant develops, which produces only a staminate 

 inflorescence. Even under ordinary growth conditions, the staminate 

 flowers originate first, and it has been suggested that the pistillate flow- 

 ers come later when the nutritive conditions are more favorable. 1 When 

 the nutritive conditions are very favorable, or when the primordia are 

 parasitized by smut, pistillate flowers may be induced in staminate 

 inflorescences. Most of the catkin-bearing trees resemble maize in that 

 the primordia of the staminate flowers develop earlier than do the pri- 

 mordia of the pistillate flowers. In Picea and in some other conifers, 

 pistillate flowers occur only on the more vigorous and better nourished 

 shoots, while the staminate flowers occur on weaker shoots. When 

 Satureja horlensis is grown in rich soil and is well illuminated, 79 per 

 cent of the flowers are monoclinous, the remainder being pistillate ; 

 in poor soil and under poor illumination only 13 per cent are monoclinous, 

 the remaining 87 per cent being pistillate only. 



It is a general belief that in dioecious plants xerophytic conditions (or conditions 

 of food impoverishment) facilitate the development of staminate plants. The ex- 

 periments cited above favor this view, but there certainly are other factors con- 

 cerned. For example, the hemp (Cannabis saliva), which is a representative 

 dioecious plant, may grow in rich, alluvial soil, where it displays great vigor, or in 

 dry and sterile waste soil, where the plants are weak and impoverished, but in all 

 cases both staminale and pistillate plants are found, if there is a large number of 

 individuals. It has been claimed that in spinach certain salts (as those of sodium 

 and calcium) favor the development of staminate plants, while other salts (as those 

 of potassium or phosphates) favor the development of pistillate plants, but this is 

 very doubtful. It has been suggested also that culture solutions which have a high 

 osmotic pressure tend to favor the development of an unusually large number of 



1 The opposite condition is seen in Humulus, in which pistillate plants sometimes 

 develop staminate flowers late in summer. 



