Image of the month (January 2013)

Pine pollen grain with wings

This image is from the website: http://www.geo.arizona.edu/palynology/

A released pine pollen grain consists of four cells, unlike in angiosperms, where in it contains either two or three cells:

a. Two prothallial cells

b. One generative cell

c. One tube cell

This four celled pollen grain is shed from the microstrobilus (male cone) in spring of year 1. The pollen grain is carried by wind to an immature megastrobilus (female cone). The pollen in the wind is caught and held by a sticky secretion from the immature female cone. As the secretion dries up, the pollen is drawn into the micropyle. Thus, unlike in angiosperms, in gymnosperms, pollination occurs before megasporogenesis (and consequently, megagametogenesis) is completed. In the ovule micropyle, pollen germinates producing a pollen tube which grows a very small distance to reach the egg and fertilize it by spring of year 2.

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