32 BOTANICAL GAZETTE [JULY 
appears within the first-formed pectic stratum; but the latter, 
now properly a middle lamella, continues to increase in thick- 
ness even after the appearance of the non-pectic layer. The 
substance of the middle lamella, now, is not rigid, but is more 
or less plastic, or even, as Dippel believes, soluble under certain 
conditions of development. Its solubility may lead at times to 
its partial absorption by the rest of the wall. More often, prob- 
ably, its mass is increased by further secretion of pectic material 
from the adjoining cells into the spaces formed by their round- 
ing up. The plastic nature of the whole layer allows the modi- 
fication of its shape, also, as the cells round up and as more 
resistent layers are deposited against it, leading to its being 
massed at the corners and pressed out to a thinner layer at the 
sides. The pressure may be in certain cases so great at the 
corners as to increase the density at those places; but in gen- 
eral, as the cells round up and draw away from the corners, the 
pressure there is relaxed, and, if new material is not deposited, 
the substance becomes less dense, perhaps swelling by the 
absorption of water. Or it may be that already the middle 
lamella has split, and the cleft has been filled by the exudation 
into it of a pectic fluid, which is later to be reabsorbed. At any 
rate, sooner or later the tension incident to the rounding-up 
process brings about a split in the middle lamella, and an inter- 
cellular space is formed. In case a Zwischensubstanz was pres- 
ent, a splitting at this time is unnecessary, and the intercellular 
space results from the absorption of this substance. If the sub- 
stance of the middle lamella is sufficiently rigid, the intercellular 
space remains angular; but if the material still retains some 
plasticity, or if it undergoes a chemical change upon its expo- 
sure at the newly formed surface, the angles may be rounded, 
or may disappear entirely, leaving a circular or elliptical space. 
The chemical’ changes effected in the middle lamella after the 
cell has attained its final form consist in the conversion of pectic 
acid into insoluble pectates, or even, as in the cork cells of Tilia, 
its replacement by entirely different substances. 
It seems to me plain that the term ‘ middle lamella,” 
