VENATION AND SENESCENCE OF POLYEMBRYONIC CITRUS PLANTS 323 
Opposed to this is the fact that Benedict (1915) reports an evident 
difference in cases in which the difference in age was only approximately 
three years. A great deal of his data are derived from plants showing an 
age difference of no more than ten to twenty years. 
2. It is possible that in Citrus grandis the venation character is not 
correlated with age. Yet Benedict found such a correlation in twelve 
species of woody plants, and he states that he believes it to be of quite 
universal application. 
A third possible explanation is given in the discussion of results. 
N UCLEO-CYTOPLASMIC RaTIO 
Inasmuch as the polyembryonic seedlings showed no differences from 
a morphological standpoint, it seemed advisable to try one other test for 
senescence — the nucleo-cytoplasmic ratio. 
To Hertwig (1903) and Minot (1908) we are indebted for the hypothesis 
that there is a definite correlation between the relative volume of nucleus 
and the volume of cytoplasm in cells as an organism passes from youth to 
old age. Minot holds that the volume of the nucleus is much larger in 
proportion to the volume of cytoplasm in cells of a young organism, and 
that, as age advances, the volume of cytoplasm gradually increases with a 
corresponding decrease in the volume of the nucleus. 
Hertwig held a view diametrically opposed to that of Minot. 
Conklin (1912) contends that such a ratio does not hold even in cells 
of the same tissue, for the ratio varies with the different phases of mitosis. 
The meristematic tissue of the root tips of polyembryonic seedlings 
was used. The seeds were germinated in pure culture, in test tubes on agar. 
They were first sterilized by removing the outer husk and allowing the naked 
seeds to stand in 7 percent hypochlorite of lime for five hours. 
The hypocotyls were allowed to grow until they were from ten to fifteen 
millimeters long. At this time the polyembryonic seedlings from single 
seeds showed differences in length of the hypocotyls in many cases. Those 
seedlings which showed this difference were the only ones used in this study, 
and as in the previous test the larger one was termed A and the other B. 
Treatment. The root tips were killed in i percent chrom-acetic acid 
solution. They were allowed to remain in this solution for twenty-four 
hours, at the end of which time they were washed in running water for an 
equal period. The subsequent treatment for fixing and clearing is that 
recommended by Chamberlain (1915) for root tips. The sections were cut 
longitudinally 10 11 in thickness and stained with Heidenhain's iron-alum 
haematoxylin. 
All determinations of the nuclear and cell diameters were made under 
the oil immersion objective. Inasmuch as a very rapid vacuolization of the 
cytoplasm occurs as the cells become more and more differentiated, no 
