jan. 20,1923 Structure of the Pericarp of Johnson Grass Seed 
213 
membrane in question more readily than most solutes, but Collins con* 
eludes that the barley grain does not appear to possess perfect imperme¬ 
ability to any solute. 
To test the resistance of the coat structures of Johnson grass caryopses 
to the penetration of iodin, caryopses of the lot which had proved to be 
most resistant to the action of chromic acid and of Javelle water (San 
Antonio, 1916, Table III and IV) were immersed in a very dilute iodin 
potassium iodid (I 2 KI) solution for three days and then tested for 
germination or sectioned for microscopic examination. Since both 
Johnson grass and Sudan grass caryopses usually contain starch in the 
embryos, the penetration of iodin solution in these can be observed to 
better advantage than in the cereals with their starchless embryos. 
About 4 per cent germinated very weakly after scratching of the embryo 
with a needle. In these the needle wound healed, with darkening of the 
surrounding cells, much as in normal untreated caryopses. In all of 
these germinable caryopses the iodin had penetrated the coat structures 
and stained the underlying starch, either locally in small patches of the 
endosperm, or less frequently around the periphery of nearly the entire 
endosperm; but in none of them was any of the starch in the embryo 
stained. In some of them small areas of the embryo or of the endosperm 
portions underlying the scutellum stained pinkish brown, possibly from 
the penetration of iodin unaccompanied by the potassium ions which are 
necessary for the formation of the blue starch-iodin combination. 
In the majority of the caryopses which failed to germinate, both of 
this lot and of No. 37001 treated at another time both in air-dry con¬ 
dition and after soaking in water, the iodin entered and stained the 
starch most promptly and abundantly along the margins of the scutellum, 
staining both endosperm and embryo starch. The stained areas gradu¬ 
ally spread to cover the whole adjoining surfaces of the endosperm and 
the scutellum. From thence the stained areas advanced to the central 
organs of the embryo, and along the periphery of the endosperm from 
the under surface of the scutellum toward the distal end of the caryopsis, 
thus paralleling in general the course described by Collins for barley. 
The evidence seems to indicate that the iodin entered most readily 
through the hilar orifice, or the micropyle, or both, passing at once 
around the caryopsis and in a distal direction along the inner surfaces of 
the aleurone layer and of the scutellum. The passage of the iodin to the 
middle part of the proximal end of the scutellum and to the adjoining 
starchy endosperm cells seemed to be retarded by the intervening large, 
empty endosperm cells lying between the hilar orifice, mircopyle, and 
proximal end of the scutellum, so that the distal end of the scutellum, 
though farther removed from the hilar region, became stained sooner 
than the proximal end. There were frequent undoubted cases also of 
local penetration of the iodin in some abundance through the uninjured 
integument. The iodin always passed only very slowly toward the center 
of the endosperm in a radial direction. In the periphery of the endos¬ 
perm the extreme distal portion was almost invariably the last to stain. 
The penetration even around the embryo was not rapid enough to prevent 
the germination while in the iodin solution of about 16 per cent of one 
lot of caryopses which had previously been soaked in water in an ice¬ 
box. Of course germination proceeded only to the extent of breaking the 
coverings over the embryo, after which the embryos were promptly killed. 
Sudan grass, wheat, and dent corn caryopses were soaked in weak 
I 2 KI solution and examined for comparison with Johnson grass caryopses. 
