AMERICAN 
25 1919 
JOURNAL OF BOTANY 
Vol. VI October, 1919 No. 8 
VENATION AND SENESCENCE OF POLYEMBRYONIC CITRUS 
PLANTS 
M. R. Ensign 
The evidence for senescence in plants has until recently been nil, 
Benedict (191 5) has, however, brought forth a considerable amount of 
data indicating that the venation in leaves of woody plants is greatly 
influenced by the age of the plant. As the plant passes from youth to old 
age, the time being reckoned since its origin from seed, the areas of leaf 
tissue enclosed by the smallest branches of the fibrovascular bundles become 
relatively smaller. These he terms vein-islets. In other words, the pro- 
portion of fibrovascular tissue in the leaves increases with the age of the 
plant. Propagation by cuttings, furthermore, does not alter at all the 
venation in the leaf. It is only when reproduction is by seed that a juvenile 
venation is obtained. 
In view of this fact it seems desirable to ascertain whether or not a 
seedling produced apogamously shows any signs of senescence as compared 
with a seedling produced from a fertilized egg. Fortunately we have in 
various species of Citrus a phenomenon which makes these plants desirable 
for such an investigation. A single seed may produce more than one seed- 
ling — a condition known as polyembryony. One of the embryos is pro- 
duced as a result of sexual fusion, the additional embryo or embryos by 
budding of the nucellus. Therefore if any differences are found in the 
seedlings or plants produced from a single seed, some evidence would be 
available indicative of the significance of sexual and apogamous reproduc- 
tion in rejuvenation processes. 
Polyembryony 
The term polyembryony means, of course, the presence of many embryos 
in a seed. This condition is not altogether unusual, yet the great majority 
of seeds produce upon germination but a single plant, i,e.^ they are not 
polyembryonic. 
Strasburger (1878), who made an extensive and detailed study of this 
phenomenon from the cytological and morphological standpoint, names the 
following plants which exhibit polyembryony: Santalum album, Sinningia 
[The Journal for July (6: 273-310) was issued Aug. 16, 1919.] 
311 
