a history of science-4-第17章

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Lister　himself；　who　had　become　an　eager　interrogator　of　the　instrument　he　had　perfected；　made　many　important　discoveries；　the　most　notable　being　his　final　settlement　of　the　long…mooted　question　as　to　the　true　form　of　the　red　corpuscles　of　the　human　blood。　In　reality；　as　everybody　knows　nowadays；　these　are　biconcave　disks；　but　owing　to　their　peculiar　figure　it　is　easily　possible　to　misinterpret　the　appearances　they　present　when　seen　through　a　poor　lens；　and　though　Dr。　Thomas　Young　and　various　other　observers　had　come　very　near　the　truth　regarding　them；　unanimity　of　opinion　was　possible　only　after　the　verdict　of　the　perfected　microscope　was　given。

These　blood　corpuscles　are　so　infinitesimal　in　size　that　something　like　five　millions　of　them　are　found　in　each　cubic　millimetre　of　the　blood；　yet　they　are　isolated　particles；　each　having；　so　to　speak；　its　own　personality。　　This；　of　course；　had　been　known　to　microscopists　since　the　days　of　the　earliest　lenses。　It　had　been　noticed；　too；　by　here　and　there　an　observer；　that　certain　of　the　solid　tissues　seemed　to　present　something　of　a　granular　texture；　as　if　they；　too；　in　their　ultimate　constitution；　were　made　up　of　particles。　　And　now；　as　better　and　better　lenses　were　constructed；　this　idea　gained　ground　constantly；　though　for　a　time　no　one　saw　its　full　significance。　In　the　case　of　vegetable　tissues；　indeed；　the　fact　that　little　particles　encased　a　membranous　covering；　and　called　cells；　are　the　ultimate　visible　units　of　structure　had　long　been　known。　But　it　was　supposed　that　animal　tissues　differed　radically　from　this　construction。　　The　elementary　particles　of　vegetables　〃were　regarded　to　a　certain　extent　as　individuals　which　composed　the　entire　plant；　while；　on　the　other　hand；　no　such　view　was　taken　of　the　elementary　parts　of　animals。〃


ROBERT　BROWN　AND　THE　CELL　NUCLEUS

In　the　year　1833　a　further　insight　into　the　nature　of　the　ultimate　particles　of　plants　was　gained　through　the　observation　of　the　English　microscopist　Robert　Brown；　who；　in　the　course　of　his　microscopic　studies　of　the　epidermis　of　orchids；　discovered　in　the　cells　〃an　opaque　spot；〃　which　he　named　the　nucleus。　　Doubtless　the　same　〃spot〃　had　been　seen　often　enough　before　by　other　observers；　but　Brown　was　the　first　to　recognize　it　as　a　component　part　of　the　vegetable　cell　and　to　give　it　a　name。


〃I　shall　conclude　my　observations　on　Orchideae；〃　said　Brown；　〃with　a　notice　of　some　points　of　their　general　structure；　which　chiefly　relate　to　the　cellular　tissue。　　In　each　cell　of　the　epidermis　of　a　great　part　of　this　family；　especially　of　those　with　membranous　leaves；　a　single　circular　areola；　generally　somewhat　more　opaque　than；　the　membrane　of　the　cell；　is　observable。　This　areola；　which　is　more　or　less　distinctly　granular；　is　slightly　convex；　and　although　it　seems　to　be　on　the　surface　is　in　reality　covered　by　the　outer　lamina　of　the　cell。　There　is　no　regularity　as　to　its　place　in　the　cell；　it　is　not　unfrequently；　however；　central　or　nearly　so。

〃As　only　one　areola　belongs　to　each　cell；　and　as　in　many　cases　where　it　exists　in　the　common　cells　of　the　epidermis；　it　is　also　visible　in　the　cutaneous　glands　or　stomata；　and　in　these　is　always　doubleone　being　on　each　side　of　the　limbit　is　highly　probable　that　the　cutaneous　gland　is　in　all　cases　composed　of　two　cells　of　peculiar　form；　the　line　of　union　being　the　longitudinal　axis　of　the　disk　or　pore。

〃This　areola；　or　nucleus　of　the　cell　as　perhaps　it　might　be　termed；　is　not　confined　to　the　epidermis；　being　also　found；　not　only　in　the　pubescence　of　the　surface；　particularly　when　jointed；　as　in　cypripedium；　but　in　many　cases　in　the　parenchyma　or　internal　cells　of　the　tissue；　especially　when　these　are　free　from　the　deposition　of　granular　matter。

〃In　the　compressed　cells　of　the　epidermis　the　nucleus　is　in　a　corresponding　degree　flattened；　but　in　the　internal　tissue　it　is　often　nearly　spherical；　more　or　less　firmly　adhering　to　one　of　the　walls；　and　projecting　into　the　cavity　of　the　cell。　　In　this　state　it　may　not　unfrequently　be　found。　in　the　substance　of　the　column　and　in　that　of　the　perianthium。

〃The　nucleus　is　manifest　also　in　the　tissue　of　the　stigma；　where　in　accordance　with　the　compression　of　the　utriculi；　it　has　an　intermediate　form；　being　neither　so　much　flattened　as　in　the　epidermis　nor　so　convex　as　it　is　in　the　internal　tissue　of　the　column。

〃I　may　here　remark　that　I　am　acquainted　with　one　case　of　apparent　exception　to　the　nucleus　being　solitary　in　each　utriculus　or　cellnamely；　in　Bletia　Tankervilliae。　　In　the　utriculi　of　the　stigma　of　this　plant；　I　have　generally；　though　not　always；　found　a　second　areola　apparently　on　the　surface；　and　composed　of　much　larger　granules　than　the　ordinary　nucleus；　which　is　formed　of　very　minute　granular　matter；　and　seems　to　be　deep　seated。

〃Mr。　Bauer　has　represented　the　tissue　of　the　stigma；　in　the　species　of　Bletia；　both　before　and；　as　he　believes；　after　impregnation；　and　in　the　latter　state　the　utriculi　are　marked　with　from　one　to　three　areolae　of　similar　appearance。

〃The　nucleus　may　even　be　supposed　to　exist　in　the　pollen　of　this　family。　In　the　early　stages　of　its　formation；　at　least　a　minute　areola　is　of　ten　visible　in　the　simple　grain；　and　in　each　of　the　constituent　parts　of　cells　of　the　compound　grain。　　But　these　areolae　may　perhaps　rather　be　considered　as　merely　the　points　of　production　of　the　tubes。

〃This　nucleus　of　the　cell　is　not　confined　to　orchideae；　but　is　equally　manifest　in　many　other　monocotyledonous　families；　and　I　have　even　found　it；　hitherto　however　in　very　few　cases；　in　the　epidermis　of　dicotyledonous　plants；　though　in　this　primary　division　it　may　perhaps　be　said　to　exist　in　the　early　stages　of　development　of　the　pollen。　Among　monocotyledons；　the　orders　in　which　it　is　most　remarkable　are　Liliaceae；　Hemerocallideae；　Asphodeleae；　Irideae；　and　Commelineae。

〃In　some　plants　belonging　to　this　last…mentioned　family；　especially　in　Tradascantia　virginica；　and　several　nearly　related　species；　it　is　uncommonly　distinct；　not　in　the　epidermis　and　in　the　jointed　hairs　of　the　filaments；　but　in　the　tissue　of　the　stigma；　in　the　cells　of　the　ovulum　even　before　impregnation；　and　in　all　the　stages　of　formation　of　the　grains　of　pollen；　the　evolution　of　which　is　so　remarkable　in　tradascantia。

〃The　few　indications　of　the　presence　of　this　nucleus；　or　areola；　that　I　have　hitherto　met　with　in　the　publications　of　botanists　are　chiefly　in　some　figures　of　epidermis；　in　the　recent　works　of　Meyen　and　Purkinje；　and　in　one　case；　in　M。　Adolphe　Broigniart's　memoir　on　the　structure　of　leaves。　　But　so　little　importance　seems　to　be　attached　to　it　that　the　appearance　is　not　always　referred　to　in　the　explanations　of　the　figures　in　which　it　is　represented。　Mr。　Bauer；　however；　who　has　also　figured　it　in　the　utriculi　of　the　stigma　of　Bletia　Tankervilliae　has　more　particularly　noticed　it；　and　seems　to　consider　it　as　only　visible　after　impregnation。〃＇2＇


SCHLEIDEN　AND　SCHWANN　AND　THE　CELL　THEORY

That　this　newly　recognized　structure　must　be　important　in　the　economy　of　the　cell　was　recognized　by　Brown　himself；　and　by　the　celebrated　German　Meyen；　who　dealt　with　it　in　his　work　on　vegetable　physiology；　published　not　long　afterwards；　but　it　remained　for　another　German；　the　professor　of　botany　in　the　University　of　Jena；　Dr。　M。　J。　Schleiden；　to　bring　the　nucleus　to　popular　attention；　and　to　assert　its　all…importance　in　the　economy　of　the　cell。

Schleiden　freely　acknowledged　his　indebtedness　to　Brown　for　first　knowledge　of　the　nucleus；　but　he　soon　carried　his　studies　of　that　structure　far　beyond　those　of　its　discoverer。　He　came　to　believe　that　the　nucleus　is　really　the　most　important　portion　of　the　cell；　in　that　it　is　the　original　structure　from　which　the　remainder　of　the　cell　is　developed。　Hence　he　named　it　the　cytoblast。　　He　outlined　his　views　in　an　epochal　paper　published　in　Muller's　Archives　in　1838；　under　title　of　〃Beitrage　zur　Phytogenesis。〃　　This　paper　is　in　itself　of　value；　yet　the　most　important　outgrowth　of　Schleiden's　observations　of　the　nucleus　did　not　spring　from　his　own　labors；　but　from　those　of　a　friend　to　whom　he　mentioned　his　discoveries　the　year　previous　to　their　publication。　This　friend　was　Dr。　Theodor　Schwann；　professor　of　physiology　in　the　University　of　Louvain。

At　the　moment　when　these　observations　were　communicated　to　him　Schwann　was　puzzling　over　certain　details　of　animal　histology　which　he　could　not　clearly　explain。　　His　great　teacher；　Johannes　Muller；　had　called　attention　to　the　strange　resemblance　to　vegetable　cells　shown　by　certain　cells　of　the　chorda　dorsalis　（the　embryonic　cord　from　which　the　spinal　column　is　developed）；　and　Schwann　himself　had　discovered　a　corresponding　similarity　in　the　branchial　cartilage　of　a　tadpole。　　Then；　too；　the　researches　of　Friedrich　Henle　had　shown　that　the　particles　that　make　up　the　epidermis　of　animals　are　very　cell…like　in　appearance。　Indeed；　the　cell…like　character　of　certain　animal　tissues　had　come　to　be　matter　of　common　note　among　students　of　minute　anatomy。　Schwann　felt　that　this　similarity　could　not　be　mere　coincidence；　but　he　had　gained　no　clew　to　further　insight　until　Schleiden　called　his　attention　to　the　nucleus。　　Then　at　once　he　reasoned　that　if　there　really　is　the　correspondence　between　vegetable　and　animal　tissues　that　he　suspected；　and　if　the　nucleus　is　so　important　in　the　vegetable　cell　as　Schleiden　believed；　the　nucleus　should　also　be　found　in　the　ultimate　particles　of　animal　tissues。

Schwann's　researches　soon　showed　the　entire　correctness　of　this　assumption。　A　closer　study　of　animal　tissues　under　the　microscope　showed；　particularly　in　the　case　of　embryonic　tissues；　that　〃opaque　spots〃　such　as　Schleiden　described　are　really　to　be　found　there　in　abundanceforming；　indeed；　a　most　characteristic　phase　of　the　structure。　The　location　of　these　nuclei　at　comparatively　regular　intervals　suggested　that　they　are　found　in　definite　compartments　of　the　tissue；　as　Schleiden　had　shown　to　be　the　case　with　vegetables；　indeed；　the　walls　that　separated　such　cell…like　compartments　one　from　another　were　in　some　cases　visible。　Particularly　was　this　found　to　be　the　case　with　embryonic　tissues；　and　the　study　of　these　soon　convinced　Schwann　that　his　original　surmise　had　been　correct；　and　that　all　animal　tissues　are　in　their　incipiency　composed　of　particles　not　unlike　the　ultimate　particles　of　vegetables　in　short；　of　what　the　botanists　termed　cells。　　Adopting　this　name；　Schwann　propounded　what　soon　became　famous　as　his　cell　theory；　under　title　of　Mikroskopische　Untersuchungen　uber　die　Ubereinstimmung　in　der　Structur　und　dent　Wachsthum　der　Thiere　und　Pflanzen。　　So　expeditious　had　been　his　work　that　this　book　was　published　early　in　1839；　only　a　few　months　after　the　appearance　of　Schleiden's　paper。

As　the　title　suggests；　the　main　idea　that　actuated　Schwann　was　to　unify　vegetable　and　animal　tissues。　Accepting　cell…structure　as　the　basis　of　all　vegetable　tissues；　he　sought　to　show　that　the　same　is　true　of　animal　tissues；　all　the　seeming　diversities　of　fibre　being　but　the　alteration　and　development　of　what　were　originally　simple　cells。　And　by　cell　Schwann　meant；　as　did　Schleiden　also；　what　the　word　ordinarily　impliesa　cavity　walled　in　on　all　sides。　He　conceived　that　the　ultimate　constituents　of　all　tissu