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Eastern cottonwood (Populus deltoides

Q. Why are there many poplars, aspens and cottonwoods in China, India and Indonesia?
A. Because those are very populous places.

Cottonwoods, poplars and aspens (genus Populus) are in the willow family, Salicaceae. Like willows, they are dioecious (having separate male and female individuals) and, in a pattern that is odd for wind-pollinated trees, not only the male but also the female flowers are numerous, and disposed along an elongate drooping catkin-like inflorescence. (Since the individual flowers are stalked it can't be correctly called a catkin because a catkin is a drooping reduced spike, whereas the cottonwood thing is a drooping reduced raceme. I'm going to call them catkins anyway, because most people do.) 

Cottonwoods flower early in spring, and do so profusely. The males are especially conspicuous, as the staminate catkins are plump finger-like appendages festooning the branches (whereas the pistillate catkins are slender finger-like appendages). Along the CS&X rail line where it crosses Rte. 98 in Caledonia, Marion County, several eastern cottonwoods grow. In April-June 2006 I folowed the progress of cottonwood reproduction there, on two trees several hundred feet apart.
eastern cottonwood

Male eastern cottonwood, April 20, 2006, Caledonia, Marion County, OH.

The male catkins produce many petal-less flowers, each of which has numerous stamens. There are female flowers per catkin. Each consists of a single 1-chambered ovary with fairly elongate stigmas that present a large surface area to catch pollen.

cottonwood male catkin

Male cottonwood catkins, April 18, 2006, Caledonia, Marion County, OH.

cottonwood pistillate catkin

Female cottonwood catkin, April 18, 2006, Caledonia, Marion County

A week after full flowering, the stamens have released their pollen into the wind and will soon fall off the tree. Meanwhile, on a female tree the ovaries have begun to plumpen (plumpen?) and the stigmas are withering a little.   

cottonwod staminate catkincottonwood pistillate catkin

Cottonwood catkins approx. one week post-flowering. Left: staminate. Right: pistillate. April 27, 2006,  
cottonwood catkincottonwood catkin

Developing fruits of eastern cottonwood. Left: May 4, 2006. Right May 11, 2006, Caledonia, OH.

By late-May/early June the capsles burst open to release many minute seeds that are each surrounded by a tuft of cottony hairs. 

cottonwood fruits

Eastern cottonwood fruits about to burst, June 5, 2006, Caledonia, OH.

cottonwood cotton   

Eastern cottonwood in full flurry, May 28, 2006, Columbus OH (about 44 miles s. of the Caledonia location).