370 University of California Publications in Botany [Vol. 5 



which in average cases measured about 20/x in diameter in the flower 

 and about 40/* in the fruit. In the second place, it is due to four or 

 five divisions of the cambium layer. This second factor in the increase 

 in size of the pedicel becomes evident when a count is made of the cells 

 between the phloem and trachea?, the result giving approximately six 

 cells in the flower and eleven in the fruit. 



The increase in size of the pedicel of Lycopersicum, which is much 

 more prominent than the increase in Nicotiana, can be explained in 

 the same manner. In the former the increase in size, which in this 

 case takes place almost entirely distal to the groove, may proceed to 

 such an extent that the diameter of the pedicel of the fruit is two or 

 three times that of the flower at anthesis. A measurement of the 

 cortical cells in cross-section gave on the average 10/i in the flower and 

 28(i in the fruit. In this case only two or three divisions of the 

 cambium occur ; the cells resulting immediately show lignification. 



The next subject of consideration is the development of mechanical 

 tissue in the pedicel of Nicotiana and its relation to abscission. It will 

 be remembered that there was no mechanical tissue noted in the 

 pedicels of buds and flowers. Parallel with the development of the 

 fruit, however, a continuous ring of mechanical tissue appears in the 

 xylem of the pedicel. This mechanical tissue is evidently the result 

 of a gradual lignification of the cells of the cambium and the outside 

 portion of the xylem parenchyma. There is thus formed a continuous 

 sheath of what may best be called wood-fibre tissue, in the form of a 

 cylinder just outside the tracheal elements. These mechanical elements 

 first appear in the tissues of the pedicel five or six days after anthesis. 

 but since the lignification in these more distal tissues is merely the 

 result of the spreading upwards of the lignification in the older parts 

 of the plant, this period depends somewhat on the position of the 

 flower on the inflorescence. It was noticed in Nicotiana that the 

 wood-fibre tissue develops on both sides of the separation zone before 

 appearing in the latter, but in time it becomes continuous through 

 the separation layer. By a lignification of the cells between the two 

 points of the crescent of wood in the separation zone, there is also 

 a slight tendency to close this crescent on the ventral side. 



Since abscission has not been observed to occur in lignified cells, 

 the question at once arises whether the tough sheath of lignified cells 

 which continues through the separation layer could hold the fruit on 

 the stem even after actual abscission had occurred. Upon looking 

 over any large number of plants in the field it will at once be evident 



