THE RELATION OF AGE TO SIZE IN CERTAIN ROOT CELLS 
AND IN VEIN-ISLETS OF THE LEAVES OF 
SALIX NIGRA MARSH. 
Marjorie a. Tellefsen 
(Received for publication June 23, 192 1) 
Introduction 
Careful microscopic studies of the effect of age on plant structure are 
few. Benedict in 191 5 found that structural changes in Vitis and in certain 
other plants are closely correlated with the age of those plants. He states 
that the vein-islets in the leaves of Vitis vulpina become smaller as the vine 
becomes older. This decrease in size is due to the encroachment of vascular 
tissue. Ensign in 191 9 found that the size of the vein-islets of Citrus leaves 
is closely correlated with the maturity of those leaves. From the most 
immature to the fully matured leaves of seedlings there is a gradual increase 
in the size of vein-islets. 
Since roots of cuttings develop from meristematic cells, the writer de- 
cided to see if the age of the trees from which cuttings are made affects the 
size of the cells in the roots developed from them. It was also desired that 
further investigations be carried on with leaf tissue, to determine in par- 
ticular if the age of a tree is in any way correlated with the size of vein- 
islets in the leaves of that tree. A woody perennial, Salix nigra Marsh., 
was chosen for the investigation. 
The black willow was selected, first, because it is abundant in the region 
of Cincinnati, Ohio, where the investigations were carried on, and second, 
because of the comparative ease with which specialized tissues, i.e., roots, 
etc., will develop from the meristematic tissue of cuttings under laboratory 
conditions, thus rendering them available for study. Furthermore, the 
black willow was chosen because of its great ability to reproduce vegeta- 
tively as well as sexually. 
Because the relation of reproductive methods to rejuvenescence has not 
been fully determined, the problem has lost none of its interest. Sexual 
reproduction is generally believed to effect complete rejuvenescence of 
protoplasm. The effects of asexual propagation are still discussed. 
Among the lower forms of life many plants and animals reproduce 
asexually for a considerable length of time, and for these organisms this 
type of propagation seems to effect rejuvenescence. Some of the uni- 
cellular forms are known to reproduce in no other way. As organisms be- 
come more and more complex, sexual reproduction appears and in many 
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