May 25, 1876J 



NA TURE 



71 



to the Royal Agricultural Society, on their Slide No. III. 

 (The three free zoospores are from a drawing by De 

 Bary.) The septate aerial branches of the fungus, named 

 conidiophores are seen at A A, The characteristic vesicular 

 swellings peculiar to the potato-fungus at B B. Immature 

 conidia at c C, and mature conidla at D, the latter show- 

 ing the contents differentiating into zoospores. The zoo- 

 spores are shown free at E E. The mycelium and conidio- 

 phores of /'t^rf«(?j/><?ra itt/cstansaxe generally furnished with 

 septa (a a) but tlais character is liable to great variation. 

 The conidia are at first temiinal (g) with no swelling on the 

 thread below, but as the threads grow they push off the old 

 conidia and continue to produce new ones on each newly 

 formed apex. De Bary explains this phenomenon by 

 saying, " When the first conidium is ripe, it is pushed to 

 the side by an unequal swelling of the point to which it is 

 attached. The top of this swollen portion then begins to 

 grow in the original direction of the branch into a new 

 conical point ; and when this has reached a length equal 

 to that of a conidium, or a conidium and a half, a new 

 conidium is produced at the apex." I take this to be only 

 partially correct, for the more reasonable explanation of 

 the vesicular swellings on the threads is that the thread 

 is constantly making an effort to produce new conidia, and 

 each swelling is really an abortive contditan : each of 

 these pieces will grow in water if free, as will the immature 

 dust-like conidia (c c) the latter are being pushed off at M M. 

 On looking at point B, it will be seen that the swellings 

 there illustrated have never produced terminal conidia at 

 all, but that each successive swelling is in itself an attempt 

 .to become one. Instead of these bodies ivhen terminal 

 growing to the length of " a conidium or a conidium and a 

 half" they commonly remain the mere fourth or sixth part 

 of a conidium in length, and often less, aad never pro- 

 duce conidia. At J will be seen a double swelling : the 

 first effort of the thread fell short, and the attempt to pro- 

 duce the conidium was renewed : such double swellings 

 are common ; a terminal one occurs at K. Vesicular 

 swellings occur on all parts of the conidiophorc ; they are 

 frequent at the base, commonly irregular as at L and 

 always (to me) represent an attempt at fruit production. 



It may just be well to remark that the suggestion as to the 

 possibility of the oospores of P. infestans being ultimately 

 found on some plaxit dilierent from Solanum tuberosum is 

 very old, and that Mr. Berkeley has recently found the 

 potato-fungus growing upon the garden Petunia, this plant, 

 we believe, has not been given in any previous list, and 

 its importance inuat not be overlooked, for the Petunias 

 come from the native country of the potato, one garden 

 species even coming from Chili. 



Prof. De Bary is liOC right in his surmise that he was 

 "perhaps" the first to call attentiua to the jjerennial 

 mycelium of the potato-fungus in 1863 ; Mr. Berkeley did 

 this in 1846 and the fact has been confirmed by many 

 observers since. The subject is thoroughly old and is dis- 

 cussed in our popular books ; for instance — see vol. xiv. 

 of the "International Series" Fungi (p. 156), where Dr. 

 M. C Cooke says, "The Peronospora of the potato is thus 

 perennial by means of its mycelium." Most lungi depends 

 for their existence upon "perennial mycelium." The 

 "spawn" of the common mushroom is a good and well 

 known example. A mycelium may however be perennial 

 and yet produce oospores. 



WORTHINGTON G. SMITH 



OUR ASTRONOMICAL COLUMN 

 The Occultation of Saturn, August 7, a.m. — 

 Perhaps some observers who are provided with good tele- 

 scopes may be induced to look for the occultation of the 

 planet Saturn, on the moruing of August 7, although (in 

 the south of England) the immersion does not take place 

 until half an hour after sunrise, and at emersion Saturn 

 is only some five degrees above the south-western 

 horizon. 



Reference is made to the phenomenon here with the 

 view to illustrate the use of the method of distributing 

 predictions over a given geographical area, explained by 

 Mr. W. S. B. Woolhouse in the Companion to the Almanac 

 for 1871, as applicable to the phases of a solar eclipse, to 

 the approximate prediction of the times of immersion and 

 emersion of a star or planet in a lunar occultation, and 

 the angles on the moon's limb at which they occur, at 

 any place within the given area or very near to it. It is 

 founded upon the assumption that the value to be deter- 

 mined is a linear algebraic function of the latitude and 

 longitude of the place, for which the calculation is to be 

 made. On this assumption the time (/), of any phase, 

 &c., may be expressed thus : — 



t^c+p. L + i'. M. 

 where c, p, and q are three constants to be found. 



If now direct calculations of the particulars of any 

 phenomenon be made for three places moderately distant 

 as Greenwich, Dublin, and Edinburgh, the constants will 

 be determined by the substitution of the results, which 

 supply the three equations of condition necessary. If the 

 difference Greenwich — Dublin be caUed h, and Greenwich 

 —Edinburgh k, then, as calculated by Mr. Woolhouse : — 

 p = o'i425 h - o'284oZ' 

 q — 0"050i4/< - 002137-^ 

 c =■ G - i'4772>6. 

 G being the result of the computation for Greenwich. 

 Also L is latitude — 50°, expressed in degrees and decimals. 

 And M is longitude from Greenwich, + if east, - if west, 

 in minutes of time and decimals. 



Applying this method to the occultation of Saturn we 

 have, by direct computation for Greenwich, Dublin, and 

 Edinburgh (astronomical times at Greenwich, Aug. 6) : — 



Almanac, and the angles expressed as usual in that 

 work. 



Thus we find for Greenwich time of immersion and 

 emersion at any place in this country, and for the angles 

 on the moon's limb from north point : — 



h. m. 

 Immersion ... Aug. 6 17 905 - i 03 L + 02 1 M. 

 Emersion ... ,, 18 3 24 - 010 L + 002 M. 



Angle Imm 903 -I- 2 "9 L-03 M. 



Angle Em 336-3 -3-5 L + 02 M. 



The differences between the reiults of these equations 

 and direct calculations for Exeter and Liverpool are : — 



Exeter. Liverpool. 



Immersion 

 Emersion . 



- 0-2 

 + o-i 



+ d-2 

 - 03 



- 0-2 

 + o-i 



Angle Imm +03 ... 



Angle Em +0-1 ... 



In this manner have been derived the following parti- 

 culars, as regards the occultation in question, which will 

 illustrate the applicability of Mr. Woolhouse's method to 

 such phenomena : — 



G.M.T. G.M.T. Angles ft om N. point 



of Immersion. of Emersion. Imm. Em. 



b. m. h. m. n o 



