2006-11-03 23:42:47 · 6 answers · asked by j_r_m_doctor 1 in Science & Mathematics ➔ Botany
Because they have died from the main tree, and during autumn there are strong winds that usually blow them off. Normally into our garden!!
2006-11-03 23:45:51 · answer #1 · answered by david l 2 · 0⤊ 0⤋
When the days get shorter and cooler the tree prepares for a winter hibernation by keeping all moisture in its truck and major branches. This prevents the leaves from getting any nourishment. They dry up and fall off.
Why does this happen? For survival. Ever been around during an early snowfall. Branches falling from the weight of snow being supported by leaves. Trees falling over from the weight.
2006-11-04 07:48:59 · answer #2 · answered by Carl 3 · 1⤊ 0⤋
In general terms, leaf senescence is a way for a deciduous plant to prepare for
winter and recycle some of the valuable and often scarce mineral nutrients,
such as nitrogen and phosphorus. Leaf senescence is also a way to to get rid of
old and photosynthetically less efficient leaves in both deciduous and
evergreen plants.
Leaf senescence is considered a genetically programmed process and is an area
of considerable recent research. The hormones ethylene and cytokinin are
involved in regulation of senescence. Cytokinin applied to leaves delays
senescence. Various enzymes are also involved. One important process in leaf
senescence is the formation of an abscission zone which seals off the vascular
connections and allows the leaf to be shed. This is important to prevent
disease-causing microbes from entering and the vascular connections from
leaking.
The development of fall leaf colors is a very noticable change that occurs
because chlorophyll synthesis stops and the current chlorophyll degrades
thereby revealing yellow carotenoids in chloroplasts that were there all along.
Anthocyanins are formed in vacuoles in some leaves giving reds and purple
shades. Anthocyanin formation in fall leaves is promoted by dry, sunny days and
cool, dry nights. Spectacular fall leaf colors are a multi-billion dollar
tourist attraction that may help prevent some forests from being logged. There
are several ideas for the development of anthocyanins including their acting as
a sunscreen, to attract animals that disperse seeds, protect from freeze
damage, or prevent water stress.
2006-11-04 13:59:00 · answer #3 · answered by t_roy_e 3 · 0⤊ 0⤋
i think carlos answer is more logical but some kind of trees can stay green during autumn
2006-11-04 13:40:04 · answer #4 · answered by source_of_love_69 3 · 0⤊ 0⤋
Because they die. Leaves cant stay on trees during winter, they will die even more!
2006-11-04 07:50:23 · answer #5 · answered by DARIA. - JOINED MAY 2006 7 · 0⤊ 0⤋
A few of them tried to float upwards, but they didn't get very far.
2006-11-04 07:52:19 · answer #6 · answered by Labsci 7 · 0⤊ 0⤋