A STUDY OF RECIPROCAL CROSSES. 9 1 



tions : Reciprocal crosses of red currant and yellow plum can be 

 made; but special care must be exercised in emasculating the 

 flowers of red currant, and the pistils must be protected from 

 injurious effects of changes of temperature and moisture. The 

 real difficulty in making these reciprocal crosses is a mechanical 

 one, and lies in the delicate structure of the pistil of the red cur- 

 rant. 



B. A STUDY OF SQUASHES. 



All the previously mentioned impossible reciprocals are those 

 between distinct species ; but with the squashes we have more 

 closely related plants, the summer crookneck and golden custard 

 being horticultural varieties of the same species, Cucu'rbita Pepo, 

 a condition which makes the problem all the more remarkable 

 and adds greatly to the interest of the inquiry for facts in the 

 general problem of impossible reciprocals. 



(i.) The field work. 



The time when this work was done, and the conditions under 

 which the material was obtained, lend latitude to the problem. 

 Some of the plants for this study were grown in the University 

 greenhouse in the winter and spring of 1903, but most of the 

 material was obtained from plants grown in the garden under 

 normal conditions of culture. Plants which were grown out-of- 

 doors were planted June 18 and began to blossom August 10. 

 Staminate flowers were the first to appear, and at this time a 

 study of the pollen of the two varieties was taken up, which will 

 be described in the histological part of this subject. 



Pollinating the Plants — The flowers of the squashes are of 

 exceedingly short duration ; the corolla remains open only one 

 day, and generally somewhat less than twenty-four hours. Such 

 short lived flowers necessitated daily attendance in the squash 

 plot in order to do the pollinating when the staminate arid pistill- 

 ate flowers were in prime condition for the operation. Note 

 should be made here of the great care that was taken in securing 

 pure fresh pollen, and applying it to the pistils that were at their 

 height in the period of receptivity. Since the flowers exist only 

 for a short time, pure pollen was easily secured by tying a string 

 over the tip of the corolla the day before the flower opened, thus 

 preventing access and mixture of pollen. Pistillate flowers were 



