Page:Popular Science Monthly Volume 39.djvu/356

342 can shore.... Kerner has seen a lake in the Tyrol so covered with pollen that the water no longer appeared blue.... Mr. Blackley found numerous pollen grains, in one instance twelve hundred, adhering to sticky slides, which were sent up to a height of from five hundred to a thousand feet by means of a kite, and then uncovered by means of a special mechanism." The so-called showers of sulphur which have at times visited various cities, notably St. Louis, are nothing but clouds of yellow pollen blown from pine or other forest trees from some distant place. Perhaps, out of millions of grains thus scattered far and wide, only a single one may be of service.

As if to compensate for this expenditure of pollen in some plants, there are others in which the amount is very limited, and where nearly every grain is made to count. These are known as cleistogamous flowers, a term applied to those which always remain in the bud. These flowers are found in plants belonging to about sixty different genera of various orders, and generally in those species which at the same time produce the normal and conspicuous flowers. These large blossoms are often sterile, and the plant must depend on the cleistogamous flowers for its seed. In the wood-sorrel (Oxalis acetosella), these flowers have each about four hundred pollen grains; the touch-me-not (Impatiens) has only two hundred and fifty, and some violets only one hundred. Even before leaving the anther cells the grains in these cases have protruded their pollen tubes; these seek the pistil and penetrate to the ovules.

It might perhaps be supposed that, as the seed can be produced so easily, all plants would have cleistogamous flowers. But here



comes into play the fact that continual close fertilization is a great detriment and not a benefit, and that it is better in the end that