1916] Goodspecd-Keudall: Partial Sterility of Nicotiana Hybrids 295 



seed exhibited, as noted, an unusnal tendency to hold their fruits to 

 dehiscence, so that, as they went completely out of bloom late in the 

 fall, large numbers of capsules were found in place and filled with 

 apparently phenospermic seed. The fact that from thousands of these 

 seeds taken from fruits of unprotected flowers less than twenty plants 

 have been raised demonstrates that successful cross-pollination in the 

 field was not the cause of non-abscission. The plants of F^ H179 from 

 which the data given in the above table were taken came up in the 

 spring of 1916 from their own roots, grew rapidly and flowered two 

 months before seedling plants were set out into the field. The be- 

 havior of the flowers was watched during this period of rapid un- 

 seasonable growth. As the season advanced, more and more flowers 

 and fruits were retained. 



Despite the range of variation in abscission of flowers on individual 

 plants and the influence of the physiological condition of the plant 

 upon the situation in general, the above table indicates that there is 

 a distinct difference in the length of time after castration or non- 

 pollination during which the parent plants and their hybrids retain 

 their flowers after anthesis. 



In the second place, it is necessary to consider the length of time 

 involved in the actual process of abscission. In this connection we 

 are concerned wdth the question whether or not the abscission layer is 

 performed in the young bud and is capable of functioning at any 

 period of later development. This is a question which it is difficult 

 to answer and concerning which it is not easy to obtain trustworthy 

 evidence. Such histological and experimental evidence as is at hand 

 seems to indicate that in the abscission of flowers and fruits of 

 Nicotiana species hybrids an area of cells across the pedicel at its base 

 is early differentiated and capable of functioning at any later time in 

 the growth of the flower which it supports. This is indicated, in the 

 first place, by the observable presence in young flower-buds of a zone 

 of smaller, isodiametric cells a few millimeters up from the point of 

 attachment of the pedicel with the main axis of inflorescence. In the 

 second place, certain species, for example, N. acuminata varieties and 

 N. glutinosa, shed their open flowers and all buds, even the most 

 minute, following a sudden change in the temperature conditions to 

 which the plant is exposed. Further, after fertilization has been 

 successfully accomplished and the capsule and the seed begins to 

 mature, abscission practically never takes place under corresponding 

 environmental conditions. If a demonstrable zone of differentiated 



