THE HORTICULTURAL SOCIETY OF NEW YORK 



but the pistil of a flower, as a rule, remains ready for pollination 

 until after the pollen is shed from some of the anthers of the 

 flower, so there is opportunity for self-pollination. Also in these 

 fruits there are flowers of all stages of development open at the 

 same time during most of the period of bloom, so there is abundant 

 opportunity for close-pollination by action of insects. As far as 

 pollination is concerned the clonal variety of such a plant may 

 be planted in solid blocks. 



But dichogamy may be so highly specialized that close-pollina- 

 tion is also restricted, as is the case with Avocados. In this plant 

 the flowers are perfect, but each flower has normally two distinct 

 periods of opening. The pistil of a flower is ready for pollination 

 at the first time the flower opens, but it is only during the second 

 period of opening, at least 24 hours later, that the pollen is shed. 

 This prevents self-pollination. But the flowers of a tree, or of 

 trees of a clonal variety, open and close together for each of the 

 two periods of opening. Flowers of a kind in the same period 

 "flock together" and this daily rhythmic synchrony restricts close- 

 pollination. It will readily be seen that if all Avocados (seedlings, 

 clonal varieties, etc.,) have the same daily sequence, cross-pollina- 

 tion among Avocados would likewise be restricted. But a sur- 

 vey of the varieties shows two main types of daily sequence. 

 In one group of varieties pollen is shed in the forenoon and 

 first-period flowers ready for pollination are open in the after- 

 noon. In the other group the behavior of flowers is in the reverse 

 order. The two types supplement each other. The time relations 

 are wonderfully adapted for cross-pollination between individuals 

 of the two groups. Solid block plantings of a clonal variety or 

 mixed planting of varieties that have the same daily sequence of 

 bloom will decrease the chances for pollination. The Avocado is 

 a conspicuous case showing that the flower behavior of a new type 

 of fruit crop and its requirements for pollination should always be 

 determined before clonal varieties are planted in solid blocks. 



WHEN SELF- AND CLOSE-POLLINATIONS DO NOT RESULT IN FRUIT 



Self-fruitlessness exists in such clonal varieties as the Napoleon 

 Cherry, the Bartlett Pear, the German Prune Plum and the 



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