116 T O R R E Y A 



slope of southern Costa Rica, at an altitude of 2800 to 2900 feet above sea- 

 level, in the middle of March, 1937. In order to learn something of the relative 

 abundance of the two classes of trees, I took a census of all those in the imme- 

 diate vicinity. Most had low branches within reach of the ground. But even 

 when all the branches of the tree were high, the flowers could be classified as 

 either in the first or second opening at any given time by examining through 

 eight-power binoculars those which stood in profile against the sky. Of 29 trees 

 which I classified, 16 belonged to Class A and 13 to Class B. It seems that 

 the two kinds of trees occur in approximately equal numbers. 



SUMMARY 



Each flower of the aguacatillo opens twice — on consecutive days. During 

 the first opening the stigma is receptive ; the stamens lie inconspicuously against 

 the petals with anthers tightly closed. During the second opening, the stamens 

 stand up prominently and the anther-valves lift to expose the pollen ; but the 

 stigma appears to be no longer receptive. Thus in its first opening the flower 

 is, in function, purely pistillate ; in its second, staminate. 



According to the behavior of their flowers, two classes of trees are recog- 

 nized. Class A comprises trees whose flowers open for the first time at dawn, 

 remain expanded with receptive stigmas for four or five hours, then close late 

 in the morning. Twenty- four hours later they open again and shed their 

 pollen. The periodicity of Class B flowers is complementary to that of Class A. 

 flowers. Their first opening, with receptive stigmas, occurs late in the morning,, 

 when Class A flowers are expanding the second time and shedding their pollen. 

 These Class B flowers open the second time early the following morning and 

 expose pollen which may be carried to Class A flowers in their first anthesis. 

 Thus, early in the morning, Class B trees supply pollen for Class A trees, 

 whereas, in the middle of the day, the Class A trees furnish pollen for those of 

 Class B. Because all the flowers on the same tree open and close almost simul- 

 taneously, self- or close-pollination appears to be rare. Insects, attracted by 

 the abundant nectar from the floral glands, are the principal agents of polli- 

 nation. 



Trees of the two classes are found in approximately equal numbers. 



San Isidro del General 

 Costa Rica 



Literature Cited 



Skutch, A. F. 1932. Observations on the flower behavior of the Avocado in Panama. 



Torreya 32: 85-94. 

 Stout. A. B. 1927. The flower behavior of avocados. Mem. New York Bot. Gard. 7: 145- 



203. 



