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FARMERS' REGISTER, 



121 



fhrpo Iivjioihesos have been advanced to account 

 for its airency iu tlie jirodactioti of tlie ve<rctablc 

 embryo. Accorilinj to one view, a germ furnish- 

 ed by tlie pollen is supposed to bede|)osited in, and 

 nourished by \hc. ovule; accordinif to another, the 

 germ is thoiiijht to be orii^inaily Ibrnied in the 

 ovule iiscif, and is merely exciteil to action bj' an 

 iniluenre derived from tlie pollen; while accord- 

 ing to the tiiinl, the endiryo is conceived to result 

 from the union of atrerm furnished by the pollen 

 with another produced by the ovule. Recent dis- 

 coveries have relidered it probable that the first 

 sup[iosition, though attended with some dilliculties, 

 will be Ibund nearest the truth. 



It was discovered by Needhani that grains of 

 pollen, moistened or thrown ujjon water, burst 

 with violence, and dischar<>'e the slighlly viscous 

 «nd turbid Huid contained in them. This effect 

 was also sometimes produced wlien the pollen fell 

 on the moist surf ice of the stiii'uia, and thence to 

 this fluitl the immediate agency in impregnation 

 was attributed by Linn;cus and contemporary 

 botanists, they in general supposing that the fluid 

 was directly conveyed down the style to the ovule 

 where the fecundating power was exerted. — 

 Whatever speculations hiight have been on the 

 eubject, the actual knowledge of naturalisis was 

 confined to the simple iact, that the application of 

 the pollen to the stigma was essential to the ti^rti- 

 lization of the ovule; all ^the information gained 

 respecting the action ol" the pollen after it has 

 reached the stigma.beingof recent date. 



The very great improvements made in the mi- 

 croscope within the last twenty or thirty years had 

 prepared the wa}' for iurther advances in science, 

 and the career of discovery in the impregnation 

 of plants was opened in 1S23 by Prof Amici of 

 Italy. In examining some grains of pollen on 

 the stigma of the Purtnlacca oleracea, or common 

 purslain, he observed that the grams had project- 

 ed li'om some part of their surface an exremely 

 slender tube, which was Ibund to consist of the 

 inner lining of the pollen-grain protruded through 

 the external coat. Amici published the result of 

 his discoveries, w^iich immediatly attracted the 

 notice of Brougniartand Hrown, the first of whom 

 published a memoir in 1837, to which we are in- 

 debted tor tlie earliest and most complete account 

 of the manner in which these tubes originate 

 and act upon the stigma. When grains of pollen 

 fall on the sligma, they are retained either by the 

 liairs with which this organ is often provided, or 

 by its^slightly humid and viscous surface. This 

 moisture they slowly absorb, and after an interval 

 varying from an hour to a day or more, the oul-er 

 coat opens by one or more coats or slits, through 

 which the higlily extensible inner membrane pro- 

 trudes like a hernial sac, and is slowly prolongeti 

 into a delicate tube. These tubes are so ex- 

 tremely attenuated, that a powerful microscope is 

 required for their examination. 



The grains of pollen produced by some plants, 

 (particularl}' the family of the CucarbiiacoE.) nve 

 known to protrude several tubes from difierent 

 points, and Amici detected as many as twenty or 

 thirty from a single grain. One of the most sing'dar 

 facts shown by the investigations ol' Brow n was, 

 that the stigma of one plant would excite the same 

 tictioninthe pollen of dillercnt species, and even 

 of plants beloniring to different ILuiiilies. Thus 

 Dr. Brown applied the pollen of a species of Ascle- 

 Vol. V-16 



pias to the sligma of an Orchideous plant, and 

 itjund these tubes produced as readily as vvhen 

 in contact with thestigma of the plant li'om which 

 the pollen was taken. Here was a clue to the 

 manner in which hybirds were llirmed; and a 

 plausible reason shown why the seeds of sucli 

 plants, when grown, did not resemble the fruit; 

 fi-om which they were taken, but showed a dispo- 

 silion to revert more or less to their original type, 

 accordinirlo ilindamenlal law of nature. 



The tubes thus produced li-om the pollen when 

 placed in contact with the stigma, penetrated its 

 substance, not however by means of any particu- 

 lar channel, but by insinuating themselves be- 

 tween the cellules, ami along the intercellular 

 passages which abound in the tissue of the stig- 

 ma and style. JM. Broiigniart was able to follow 

 them foV only a moderate distance into the tissue 

 of tJie style, where he imagined the tubes termi- 

 nated, and opening at the extremeity , discharged 

 the fluid and floating particles of the pollen grain. 

 Dr. Brown u-as more successful in tracing their 

 course. He fi)llowed the course of tlie pollen tubes 

 in several plants of Orchidea and xlsdcpiaded 

 from the stigma to the placenta, and in a single 

 instance, in an Orchideous plant, traced some ves- 

 sels or tubes of an equivocal nature quite into the 

 aperture of the oVule. Dr. Biown published an 

 account of these discoveries in 18.31, and remarks 

 that Mr. Elliott, in his Eo!an3'of South Cnrolina 

 and Georgia, had observed these cords or fibres m 

 the Ascelpias, but it is probable their origin or of- 

 fice was not suspected. 



Such was the state of knowlegde respecting 

 tho impregnation of plants, when M. Corda, a 

 member of the Imperial Academy, commenced 

 his observations. Dr. Brown had traced the pol- 

 len tubes to the niouth of the ovules, and M. 

 Corda devoted himself to determine the interest- 

 ing inquiry as to their farther progress and termi- 

 nation. As subjects of examination, he selected 

 the Chnijerm or fir tribe, in which th.e ovules are 

 naked,, and impregnated by immediate contact 

 with the pollen, consequently offering great faci^ 

 lities for the investiiration. Aided by powerful 

 instruments, and conducting the processes with 

 great skill and perseverance, M. Corda was able 

 at last to trace the pollen tubes into the ovule 

 ilself, where, the fluid of the pollen gave tne first 

 impluse to the formation and developement of the 

 embryo. The lollowing is the summary of the 

 discoveries made by iVl. Corda, translated from 

 his memoir by Dr. Gray, and v.'hich in the Coid- 

 /trffi establish these positions: — 



"1. The pollen tube penetrates into the micro- 

 pvle; (exostome,) and in Pinus the |)ol!en grains 

 fall directly into it, whence the impregnation is 

 immediate. 



2. The pollen tube passes through the exos- 

 tome into the endostome, passes through the ca- 

 vity of the secundine, and arrives at 



3. The nucule or ovule ; extends through ilie 

 endostome into its cavitj^, and 



4. By the injection of the fluid contained in the 

 pollen grains in the bottom of the nucule gives 

 the first kcim (germ) to the formation of tlie 

 embryo. 



5. The formation and development of the em- 

 bryo changes the contents of the cellular tissue of 

 I he nucule, which becomes fluid, and appear to fur- 

 nish materials (or tlie growth of the embryo. 



