1915] BLODGETT—LEMNA FROND 387 
terminal bud is divided by vertical pressure and constraint, and 
appears as a pair of lateral buds except in the initial stages; but the 
axillary bud is developed in its normal position, behind the foliar 
outgrowth. 
Homologies of the Lemna frond 
The Lemna frond is a structure of reduced character, especially 
adapted to vegetative propagation under particular conditions. 
The two forces which act upon the plant as possible determining 
factors in the resulting structure are the tension of the water film and 
the buoyant effect of the liquid. The film tension tends to atten- 
uate in all directions any elastic structure floating upon it, while 
the upward thrust of buoyancy tends to lift any deep-lying portion 
into the same plane with the thinner parts. The thin frond responds 
to the conditions imposed, and may be considered as a result of the 
forces involved. The roots act as drags or brakes against too 
free movement in the water, and to return the dorsal surface to the 
air after the frond has been immersed by any sudden shock, the 
drag of the projecting roots tending to orient the frond as it rises 
to the surface by its buoyancy. Inverted plants of Spirodela 
will thus orient themselves in a 3-inch dish of water, but complete 
inversion would but rarely occur under natural conditions. 
The runner of Erythronium is a structure also of reduced nature 
and also developed to meet particular conditions. In this case 
the forces involved are the outward thrust of growing tissues and 
the resistance of the surrounding soil. The root type of structure 
is manifestly best fitted to this condition, having the region of 
elongation close to the apex of the structure. An investigation of 
the development of these runners® showed that the zone of elonga- 
tion is so located, and that the component parts of the runner are 
adapted to the conditions and factors present, namely, the offsetting 
of propagules, and their movement through soil. A longitudinal] 
section of a runner is shown in text fig. 1 (I). The bud scale form- 
ing the anterior wall of the hollow tip elongates, as do leaves, by 
basal growth; the axial portion of the runner which thrusts the 
terminal bud forward elongates by growth near the tip. The two 
’ Biopcett, F. H., Origin and development of bulbs in Erythronium. Bort. Gaz. 
50: 340-372. 1910. 
