Drjúpa döggvartár

Skrifað um July 14, 2013 · in Almennt · 188 Comments

Droparnir frá tönnum safnast saman í botni blöðkunnar. - Kjörið er fyrir fólk að reyna við þetta sem myndefni. Ljósm. ÁHB.

Dropar þessir eru til komnir vegna þess, að á kyrrum og rökum nóttum safnast vatn, sem plantan tekur upp gegnum rætur, í stóra dropa á blöðunum. Þetta kallast táramyndun (guttation). Vatnið streymir út um æðar á röndum blaða og og rennur saman í skál blaðsins og myndar þessa glitrandi dropa á vaxþöktu yfirborði blaðsins. - Kjörið er fyrir fólk að reyna við þetta sem myndefni. Ljósm. ÁHB.

 

Svo kölluð táramyndun (guttation, latína gutta, dropi) í plöntum er merkilegt fyrirbæri. Það eru ekki nema um 300 tegundir háplantna, sem sýna veruleg merki um þetta fyrirbrigði.

Vatn, sem seytlar út við táramyndun, er ekki hreint, heldur inniheldur það ýmis steinefni, lífrænar sýrur og jafnvel enzým. Þegar táradöggvar gufa síðan upp af blöðum geta þessi efni valdið smáum skemmdum á blöðunum, sem kalla mætti tárabruna.

Vatnið kemur út um sérleg augu, en ekki venjuleg loftaugu, sem eru umlukt varafrumum, heldur opum, sem kalla mætti vatnsaugu (hydathodes) og eru alltaf opin. Vatnsaugun eru oftast í blaðoddi eða á tönnum. Þetta eru op í yfirhúð en rétt undir yfirborðinu er sérlegur tróðfrumu-vefur (epithem), sem er laus í sér. Frumurnar eru ferningar með miklu millifrumurými, og þangað ná endar sumra viðaræða. Tróðfrumurnar, sem umlykja viðaræðarnar, eru af sérlegri gerð, sem einkennir frumur, sem miðla vatni. Bæði veggur og himna liggja í fellingum, sem eykur verulega yfirborð þeirra.

Við sérstakar aðstæður þegar mikill raki er í jarðvegi og lofti taka rætur plantna upp vatn í ofurmæli. Þrýstingur í rótarkerfi knýr vatnið upp og það leitar út um vatnsaugun. En málið er ekki alveg svona einfalt, því að hin stóra spurning er, hvaðan koma efnin sem finnast í vatninu. Sennilegt er, að tróðfrumurnar gefi frá sér þessi efni, en það er þó engan veginn fullljóst.

Táramyndun á sér stað að kvöldi þegar loft er kyrrt og nær rakamettað. Táradöggvarnar hverfa ekki fyrr en næsta dag, þegar loft hitnar.

Sumum kann að reynast erfitt að greina á milli döggvar (daggar) og táradöggvar, en dögg myndast þegar raki í andrúmslofti þéttist, þegar það snertir kaldan hlut, eins og húsþak.

Eins og flestir þekkja streymir vatn án afláts frá rótum og upp alla plöntuna um viðaræðar og flytur eingöngu með sér steinefni úr jarðvegi. Þetta vatn hverfur úr plöntunni á daginn við útgufun aðallega út um loftaugun. Aldrei myndast vatnsdropar við venjulega útgufun. Á næturnar, þegar táramyndunin á sér stað, hafa varafrumurnar hins vegar lokað fyrir loftaugun. Vatnsdropar þessir eru því alls ekki venjuleg dögg, sem myndast við náttfall og í þoku, eins og á stundum er haldið ranglega fram.

Eins og áður sagði, er táramyndun ekki algeng meðal plantna. Hér á landi má sjá hana á tómat-plöntum í gróðurhúsum, rósa-tegundum, ýmsum grastegundum (t.d. loðgresi, Holcus lanatus) og ekki sízt hjá maríustökkum eða öðru nafni döggblöðkum (Alchemilla).

Síðdegis á sólríkum dögum gufar vatnið síðan upp. Ljósm. ÁHB.

Síðdegis á sólríkum dögum gufar vatnið síðan upp. Ljósm. ÁHB.

Nafnið alchemilla er komið úr arabísku, alkemelych, og merkir gullgerðarlist. Skýring á því er sú, að á fyrri tíð héldu menn, að unnt væri að vinna gull úr dropum, sem safnast fyrir á blöðum plantnanna.

ÁHB / 14. júlí 2013

 

P.s. Fyrirsögn er sótt í kvæðið Blítt lætur veröldin eftir Jóhann Jónsson (1896-1932).

 

 

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