428 Miscellaneous. 



these 1 have had for more than ten years. It must be thirteen years 

 old, from my observation of the rate of growing. 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 ribs. And, in both, the tubercles 

 are arranged, with reference to one another on the adjoining ribs, in 

 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 fiagelliformis ; 

 — 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, 

 ox 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 in a 

 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 



