154 The Botanical Gazette. [April, 
the cast; as extreme examples may be mentioned Vicia, 
Dahlia, Ricinus and Pterocarya where the diameter outside 
the cast was often to that within as two to one. 
Effect of mechanical reststance 
on the duration of the period of development of cells. 
1. On the zone of elongation in roots and stems.—When 
root-tips or stem-tips of seedlings are fixed in a gypsum cast, 
the power of elongation becomes day by day reduced to nar- 
rower limits, so that when the growing point is released from 
its confining envelope, subsequent growth shows that the 
proximal limit of elongation is nearer the apex of the organ 
than formerly. Pfeffer* demonstrated this in several species. 
In the primary root of Vicia faba, for instance, where not- 
mally the elongating zone is about 10", he found this zone 
reduced to 5™™ or 6™ after two or three days ina cast. My 
own measurements have shown that in a normally growing 
primary root of Vicia faba at a temperature of 20°, the fourth 
millimeter from the apex of the root will in twelve hours have 
passed out of the segment of elongation. But Pfeffer’s root- 
tips showed elongation in the fifth or sixth millimeter after 
two or three days in casts. Thus it is evident that the effect 
of the casts was to,retard the passage of the elongating Seg 
ment into permanent tissue. : 
Analogous with this result is that obtained in my exper 
ments with Juncus and Lamium, where several very short 
shoots of the former in which tissues were undifferentiated 
were kept alive for eleven weeks in casts, and then show 
no differentiation; and in the latter the stem just behind the 
terminal bud was, in one case for twenty-five days, in another 
for forty-five days, by the same means kept from dev 
farther, except that two or three cells in the primary bundles 
slightly thickened their membranes. Meanwhile the stems 
had grown above the casts and the tissues had become mu 
better developed than within the casts. aly 
_ 2. Ondifferentiation in fundamental parenchyma.—Noe ‘lt 
in the tissue adjoining the meristem of growing points ne 
development proceed more slowly when a mechanical res! 
ance prevents expansion, but in those later change 
4 : ‘IL the same 
lly form the 
result follow. In Zea mais the cells which norma! 
 §*Pfeffer, 1. c., pp. 120 and 149. 
eloping 
a 
