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Zakaj listje iz dreves odpada

OdgovorNapisal/-a Alen » 10 nov 2005, 19:24

Prosm da mi še danes odgovorite ker se gre za oceno pri BIOLOGIJI in do jutr morm naredit Prosm :cry: :cry: :cry: :cry: :cry: [/b]
Alen
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OdgovorNapisal/-a petalo_blu » 10 nov 2005, 22:17

Tole je del teksta, ki sem ga našla na: http://ncnatural.com/wildflwr/fall/science.html -
na koncu besedilo v slovenščini (moj povzetek - upam da ok).


The largest factor in why leaves change color in the autumn is photoperiodism-the length of day and night. As the nights get longer in Autumn, the process of senescence becomes apparent through color change and the falling of leaves, leading the tree into it's winter dormancy.
...
Most notable in the process is the destruction of the chlorophyll which has been responsible for the green color of the leaves throughout the spring and summer and plant growth. As the photoperiod decreases, the plants ability to synthesize chlorophyll becomes reduced, and yellow and orange carotinoids and xanthophylls, always present within the leaf, begin to show. Water and nutrients are being drawn into the stems and away from the leaves. Additionally, senescing cells also produce other chemicals, particularly anthocyanins, responsible for red and purple colors. Some species, particularly the Oaks, contain high quantities of tannins in the leaves which are responsible for brown colors.

In preparation for winter and to prevent or minimize damage from cold, plant cells switch from production of chlorophyll for growth, to production of sugars and amino acids,...
... The process by which nutrients from the leaves are changed into other forms and drawn into the stems and roots is very efficient, and by it's end, the only remnants in the leaves will be cell walls and nutrient depleted protoplasm.

Skratka gre za to, da se v jeseni skrajšajo dnevi in dalšajo noči. Nastopi proces uničenja klorofila, ki je bil odgovoren za zeleno barvo v listih čez pomlad in poletje. Sposobnost rastline za sintezo klorofila skozi fotosintezo se zmanjša in se pokaže rumena in oranžna barva listov. Voda in hranilne snovi ne gredo več v liste. V pripravi rastline na zimo in za zmanjšati na minimum poškodbe zaradi mraza, rastlinske celice prenehajo s produkcijo klorofila za rast in začnejo proizvajat sladkorje in aminokisline.
Na omenjeni hp strani je na koncu strani še bolj konkretno napisano Zakaj listi odpadejo - je bolj strokovno (biologija) prikazano dogajanje v listih. Lahko si prebereš še ta odstavek na hp.
Srečno!
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