SKUTCH: FLOWERS OF THE AGUACATILLO 115 



plementary to that of the first. The flower buds of the trees of Class B expand 

 for the first time between ten and eleven o'clock in the morning, at the same 

 time that the flowers of the trees of Class A are opening for the second time. 

 Probably because of the higher temperatures which prevail at this hour of 

 the day, they reach their full expansion more promptly than the Class A 

 flowers at their first opening ; and their stigmas are ready to receive the pollen 

 which the Class A flowers, open for the second time, now begin to shed. 



The transfer of pollen from flower to flower is effected by the small insects 

 which come in large numbers to the abundant nectar secreted by the floral 

 glands. In flying from a tree of Class A to one of Class B, they may carry 

 the pollen grains to the latter. The flowers of Class B close at approximately 

 the same time in the afternoon as those of the other class. Their period of 

 closure is considerably shorter, for they open again at the following dawn, 

 only fifteen hours after they folded together at the end of their first opening. 

 The valves of their anthers begin promptly to rise, and before the sun has 

 appeared they are already shedding their pollen. This the insects may now 

 carry to the stigmas of the newly opened flowers of neighboring trees of 

 Class A. Thus the two kinds of aguacatillo trees reciprocate: during the early 

 morning the Class B trees supply pollen to those of Class A ; and during the 

 middle of the day, the Class A trees furnish pollen for the stigmas of the 

 flowers of their neighbors of Class B. 



The flowers of Class B make their second and final closing at the same 

 time that a new set of flowers on the same trees is opening for the first expan- 

 sion, and at the same time that the first-period flowers of the Class A trees 

 are closing and the second-period flowers of these trees are opening. The 

 interval between ten and eleven o'clock in the morning is the time of greatest 

 activity of the flowers of the aguacatillo trees, for then all of them are either 

 opening or closing, and each individual tree changes, for a period, from func- 

 tionally pistillate to functionally staminate, or the reverse. 



The opportunities for self- or close-pollination of the aguacatillo flowers 

 are very limited. Because one of two different sets of flowers on the same 

 tree opens while the other is closing, it is not impossible that the pollen from 

 a second-period flower of a Class A tree, which opens its anther-valves 

 promptly, may be carried to the stigma of a first-period flower of the same 

 tree that is slow in closing. Similarly, on a tree of Class B, lingering pollen 

 of a second-period flower may possibly be transferred to the stigma of a 

 first-period flower which has just been exposed by the separation of its petals. 

 But because of the close synchronization of the opening and closing of the 

 different sets of flowers, and because of the brief period during which the 

 transfer of pollen from anther to stigma of the flowers of the same tree is at 

 all possible, it seems unlikely that many close-pollinations occur. 



The foregoing account of the behavior of the aguacatillo flowers is based 

 upon observations made at Rivas, in the basin of El General, on the Pacific 



