Have you ever lost sleep wondering how healthy leaves decide to call it a day and fall off their parent plant? Prepare for a good night’s rest.
Plants react to shorter autumn days by producing a layer of soft, weak cells at the nodes, where leaves connect to stems.
Professional plant people call this tissue the abscission layer, from the Latin abscisus, which means to cut off. The forest floor is proof that even evergreens shed leaves, just not all at once.
A leaf stem, or petiole, is an extension of the plant’s simple circulatory system. The abscission layer is like an umbilical clamp on veins and arteries that transport water and minerals into the leaves; and, air and sugars out of the leaves.
Leaves look simple, a blade and a stem, right? Up close-and-personal, leaves are agri-businesses that employ armed bodyguards, tough, thick cells called the epidermis or skin.
There’s more than one type of plant skin. For example, holly leaves feel nail polish hard, hairy Lamb’s Ears feel downy soft, and cacti will poke you with nasty spikes. The leaf surface is covered with a layer of wax that staves-off dehydration and infection.
Did I mention that leaves breathe? Both sides of the blades contain microscopic pores called stomates. In good weather the two sausage-shaped guard cells around the pore shrink back, to let the plant exchange gases just like our lungs do. We breathe in oxygen and breathe out carbon dioxide. Plants do the opposite.
Stomates respond to light.
At dawn, the guard cells begin to shrink. This tops out around noon each day, then the cells start swelling, and shut completely by dark. Stomates close in bad weather, so plants don’t wilt from thirst.
Sandwiched between the upper and lower epidermis is the leaf mesophyll. That’s a clever word for two layers of material in the middle. The mesophyll layers contain green chloroplasts that capture sunlight and use its energy to decompose water then glue the parts to carbon dioxide molecules to make sugar for the plant to eat. Plants don’t eat fertilizers; soil microbes do.
The average leaf works hard the whole growing season. Except for resting at night, when there is no light to capture, these little factories don’t get free weekends, national holidays or paid vacations. Depending upon the plant, a leaf’s life span averages one- to three-years.
There are six basic leaf types: (1) common foliage, (2) scales that protect buds, (3) those fat leaves inside seeds, (4) spines and tendrils, (5) thick storage leaves, like kalanchoes and cacti; and, (6) bracts, which look like flowers but are not– think poinsettias and dogwoods.
Edible leaf blades include chives, greens, loose-leaf lettuces, parsley and the like.
Edible leaf petioles include celery and rhubarb.
If you like leeks and onions, you’re eating leaf-base clusters. Head lettuces, Brussels sprouts and cabbages are naked bulbs of plant leaves.
Bon apétit and throw the rest back into the garden for compost or feed the chickens!
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