34 BULLETIN 1191, U. S. DEPARTMENT OF AGRICULTURE. 
abrasions, especially the surface layer, through which it can enter. 
Fronds are not attacked until mature or nearly mature, when they 
have more of a tendency to float on the surface. The frayed and 
decaying tip of the blade affords the most common entrance. (Fig. 
15.) <A young lesion appears as a small, semicircular, dark-brown 
spot at the torn edge of the blade. This spreads very rapidly until 
it becomes 2 broad band crossing the upper end of the leaf. The 
affected tissue turns nearly black, becomes soft, and soon weathers 
off. Infection spreads rapidly to adjacent leaves, and soon after its 
first appearance the disease destroys the entire floating portion of 
the frond. In badly perforated leaves infection sometimes takes 
place in a dozen parts at once. Where growing tips float on the 
surface, as they sometimes do in heavily matted kelp, their delicate 
tissues are readily susceptible to attack. Sometimes growth 1s ar- 
rested by the young stipes becoming in- 
fected. Infections of the epidermis usu- 
ally appear as circular dots. Below the 
blackened region the color fades from 
the normal brown to dingy yellow. 
After destroying the blade, the disease 
attacks the cyst and later the stipe. 
(Fig. 16.) These parts do not blacken 
usually, but turn rusty brown and later 
white when the decayed surface layers 
dissolve off from the colorless interior 
layers. Affected fronds generally weaken 
and slough off gradually. More notice- 
able, however, is the breaking off of the 
"1g. 15.—Young leaves at the tip cysts, which sometimes drift in on the 
tacked by black tot" “© beaches in enormous numbers. Old dis- 
mantled stipes may remain attached 
until they dissolve or sink, because with the leaf blades gone there is 
nothing for wind or wave to catch. 
Why black rot does not usually cause a frond to sink becomes clear 
if we examine the structure of the wall of a cyst. The first layer or 
two of cells contain color granules, are assimilating layers, as are 
the corresponding layers in the expanded blade, and absorb water 
readily, being always filled with water in the living plant. Their 
outer surface is more or less slimy. The next layers are composed 
of colorless, closely packed, thin-walled cells, also fully charged with 
water, which make up a tissue resembling that of a summer turnip. 
The inner layer consists of felted fibers presenting a dry silky sur- 
face with loose bits of weft here and there. This inner layer keeps 
water out of the cyst. The other layers may be removed, but no 
water can enter the cyst if this inner layer is not impaired. Since 
black rot works inward, it can destroy the outer layers of a cyst 
without impairing its buoyancy. On the contrary, by destroying 
the leaves first it renders the fronds unusually buoyant. 
The new fronds sent up from the base of a diseased plant are 
usually clean and healthy until they attain the surface, although 
plants are sometimes weakened throughout, or even killed, the deli- 
cate spore-bearing tissues perishing first. The lower submerged por- 
tions, however, are not directly attacked. Since black rot becomes 
