428 Miscellaneous. 
these I have had for more than ten years. It must be thirteen years 
old, from my observation of the rate of growmg. At three years old 
it was, according to the received descriptions, ‘‘ subglobose,” &c. 
But with its growth, it has altogether lost this character. And, 
while the flowers are identical in general characters with those of the 
Cereus,—very much more identical in detail with many species of 
the Cereus than those of different received species of Cereus are with 
one another,—it is quite impossible to separate the plant itself, in 
its general habit, from any characters taken from a general survey of 
the Cereus. The plant in question stands beside a specimen of Cereus 
whose flower that of the Echinocactus very nearly resembles, and 
which is remarkably full-grown and stout. Both plants grow tall and 
straight. Both have deep straight nbs. And, in both, the tubercles 
are arranged, with reference to one another on the adjoining ribs, im 
a regular figure, the quincuncial,—a matter which will, I think, be 
remarked in all the numerously ribbed species of Cereus, and a similar 
character of regular relative arrangement in those which have only 
three or four ribs. The Echinocactus is now nearly a foot high. It 
has continually, and regularly, grown in height, but does not get any 
broader. 
1 could enlarge on some other points of character; but this letter 
has already become longer than I intended. I will only add, that 
young plants sometimes run into the long thin form of so many of 
the Cereus. I have had young shoots of Echinocactus which could 
not be distinguished from young shoots of even Cereus flagelliformis ; 
—which will be admitted to be about as extreme a comparison as 
could be made. 
Thinking that any observations which can tend to the elucidation, 
or fixation (if I may say so), of the important and interesting question 
of What is a Genus? cannot be wholly useless, I place the above very 
much at your service. 
I am, my dear Sir, very truly yours, 
W. Francis, Ph.D. J. TouLMIN SMITH. 
CAUSE OF THE POTATO DISEASE. 
The precise cause of the potato disease is still unknown; but we 
are able at least to eliminate certain presumed causes, and to prove 
where the disease begins, and how it reaches the tubers. It is pretty 
generally admitted at present that the parts of the plant exposed to 
the air are first attacked, and that their diseased state precedes that 
of the tubers, and probably causes it. The following is a rather 
curious proof that such is the case. M. de Gheldere of Thourout in 
Belgium grafted some tobacco plants upon potatoes, according to 
Tschudy’s method. Success was probable, as the Nicotiana and 
Solanum belong to the same family. The grafts did not merely take, 
a fact of itself very interesting, but the plants happening to be ma 
field of potatoes entirely attacked by the disease, the grafted stocks 
alone remained exempt. If the tubers were sound in this case, it 
can only be attributed to the presence of the leaves of tobacco not 
liable to the disease, instead of leaves of the potato itself. The fact 
