122 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
forms entirely replaces asexual propagation, while in other species both 
sexual and asexual reproduction occur. 
Because sexual reproduction causes rejuvenescence, the efficiency of 
vegetative propagation in these more complex organisms has become a 
subject for investigation. The question is especially pertinent because 
many of the higher plants, especially cultivated forms, are propagated ex- 
tensively by means of cuttings. If vegetative reproduction does not 
effectively rejuvenate protoplasm in these complex forms, plants grown 
from cuttings are as old as the plant from which the cutting was taken. 
The direct bearing of this question on the commercial propagation of certain 
cultivated plants is evident. Salix nigra Marsh., the species selected for 
the present investigation, is one in which the power of vegetative propaga- 
tion is well developed, and one in which sexual reproduction occurs regularly. 
In such a species any relation exhibited between age and cell structure is 
particularly significant. 
Collection and Care of Cuttings 
During the fall of 1917 and the spring of 1918, cuttings were made from 
trees of Salix nigra. They were tagged, numbered, and taken to the 
laboratory where they Were placed in warm water over night. The follow- 
ing day the cuttings were put in jars of tap water. Fresh water was supplied 
every few days. About a month later the cuttings were transferred to a 
galvanized iron tank of running water. A slatted arrangement made of 
wood floated on the water and served to hold the cuttings up and to keep 
them from too close contact with each other. A correct balance could 
not be determined between the required intake of water for aeration and 
the requisite temperature for root formation, so that water molds developed 
and killed the cuttings. However, a few roots were formed, and these 
were killed and imbedded for study. 
During the fall of 1918 and the early spring of 1919, cuttings were made 
from branches of recent growth of trees of Salix nigra. The age of each 
tree was determined roughly by measuring the diameter of the main trunk 
of the tree six inches above the ground. The latter distance was arbitrarily 
chosen so that measurements would be taken a uniform distance above the 
ground. The trees were then classified according to their diameter. Three 
groups were differentiated and arbitrarily distinguished by the letters A, 
B, and C. Trees less than 2}^ inches in diameter were considered to be in 
group A, those 4 to 6 inches in diameter in group B, and those 8 inches or 
more in diameter in group C. 
Cuttings were tagged and taken to the laboratory, where they were 
placed in a warm bath (38°-40° C.) for two hours to stimulate protoplasmic 
activity. When removed from the bath, some were placed in the tank of 
running water. The majority of the cuttings, however, were put in glass 
jars which were then placed either in Wardian cases in diffuse sunlight or 
