1894. ] The Influence of Mechanical Resistance. 193 
of considerable but less than normal thickness. Archangel- 
ica sativa, examined after twenty days’ growth with a cast 
around a segment of its stem, had outside the cast strong 
mechanical bast, inside the cast only thin-walled cells. Dahl- 
ia variabilis when grown with a cast around the stem de- 
velops hard bast outside the cast several weeks before it ap- 
pears within; the same is true of Ricinus communis, Forsythia 
Viridissima and Pterocarya fraxinifolia. 
In all these plants in which the delay in the formation of 
thick-walled xylem and phloem has been considered, there 
as been an actual extension of the period between the origin 
of the cells and the assumption of their permanent condition. 
This is proved by the fact that all of the species that have 
4. On the time of cork-formation.—No experiments were 
made directly on the formation of cork. The results are thus 
volhin a casts of gypsum. Whether the influence of the 
¢ aad ue to the pressure which is soon developed by the 
hd oe Confined tissues to expand, to the protective in- 
termin, i the envelope, or to some other cause is left unde- 
general "It seems probable, however, as will appear in the 
Sure oY at the Close of this paper, that itis the pres- 
In all ich is the influential factor. 
ing, cork -: € plants that have given results under this head- 
| “ormation appears more tardily within the limits of 
the Casts : 
. t eT ‘ 
Vitidissima han outside. This has proved true for Forsythia 
Major jn tst hypodermal row of cells; and for Melianthus 
of the ah ae? hellogen appears in the innermost part 
: vas ies 
. °F mechanical resistance on the permanent condition 
assumed by cells. 
of experiments to convince one that if 
It r : 
a - 'T€S No series 
fgan in which primary extension is not com- 
Tan o 
