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A TEXTBOOK OF THEORETICAL BOTANY 



or short-styled flower receives pollen produced at the same level in the 

 flower, by the anthers of the long or short stamens respectively, produced a 

 higher degree of fertility than illegitimate pollination. Such species as 

 P. officinalis, P. japonica and P. auricula (Garden Auricula) are very 

 infertile when insect visits are prevented, but completely fertile when such 

 insects are allowed, or artificial pollination permitted. He found that the 

 legitimate pollination was about one and a half times as fertile as the 

 illegitimate type. Later it W'as found by Hildebrand that when the flowers 

 are artificially self-pollinated, fertility is at a minimum. When the parents 

 are both long-styled, the offspring are also predominantly long-styled, and 

 similarly in the case of short-styled flowers. Crosses between long- and short- 

 styled forms gave a progeny with long- and short-styled flowers in approxi- 

 mately equal numbers. 



Most of the species of Primula (Fig. 1781) are pollinated by Humble 

 Bees and similar long-tongued insects, though Humble Bees have been 

 observed to perforate the corolla tube in their search for nectar. In P. 

 vulgaris, the typical heterostylous condition is w-ell seen, and it is interesting 

 to note that there are microscopic differences between the size and shape of 



Fig. 1 78 1. — Heterostylous Primula flowers, in longitudinal section. Left, 

 short-styled, thrum-eyed. Right, long-styled, pin-eyed. Legitimate 

 pollination follows the directions shown h\ the arrows. 



the stigma in the two forms, and also in the diameter of the pollen grains; 

 those of the short-styled flowers being larger than those of the long-styled 

 ones. 



Hybrids between closely allied species of Primula are not equally 

 fertile, for the sterility of the pollen increases as the affinity of the stocks 

 becomes more remote. 



The following table illustrates this point clearly: 



