Flowers on trees

Spring is bloom time for angiosperm trees (that’s trees other than conifers and ginkgoes) in temperate climates. Trees have two basic approaches to flowering, make big showy flowers and use insect pollinators, or make lots of small, inconspicuous flowers and use the wind to transport pollen. I saw both of these lately.

Staminate and pistillate (arrow) catkins of alder bloom in early spring.

Staminate and pistillate (arrow) catkins of alder bloom in early spring.

The alder was in flower in mid-March. It had formed the buds of its inflorescences last summer, so it was ready to go when the weather warmed. Trees with wind-borne pollen must bloom before their leaves bud out. The pistillate flowers form in little catkins, marked by the arrow in the photo. The bracts between the flowers persist and enlarge as the ovaries develop. When the fruits mature in the fall, and the seeds are shed, the bracts remain. They are the structures in the back that look like miniature brown pine cones. The staminate catkins are much more conspicuous in bloom, but they fall off after they have released their pollen.

Alders in most areas of the US are likely to have bloomed by now, but their cousins, the birches, are yet to flower. Birches have similar staminate inflorescences, and these also form the previous summer. The pistillate flowers are borne on an upright catkin-like inflorescence. When the fruits are mature, the bracts and the seeds are shed, leaving a bare stem. Some of the bracts may not be shed by spring, but you can easily tell a birch from an alder by the little cone-like structures on the alder. Their cousins, the hazelnuts have similar staminate catkins, but all you can see of the pistillate flowers is a cluster of tiny red threads sticking out some bracts.

The tiny red thread-like styles of a hazelnut protrude from a cluster of bracts

The tiny red thread-like styles of a hazelnut protrude from a cluster of bracts

The pear tree, on the other hand, is very showy with its white-petaled flowers. It is a member of the rose family, whose flowers have either one carpel or several carpels distinct from one another. The pear and the apple typically have five carpels. You can see the five styles and stigmas in the photo below. The flowers of cherries, plums, apricots, and almonds look a great deal like apple and pear flowers, but the stone fruits have only one carpel. The immature anthers of the flower are pink. As they mature, the anthers split and peal back, revealing the pollen. The anthers shrink and darken when they are mature.

Be sure to observe and point out blooming trees to your children this spring. They may not notice without your help. They will likely be interested in the sequence of fruit development, once they see the flowers.  

This pear flower has five green styles and stigmas.

This pear flower has five green styles and stigmas.

 

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