WHY PLANTS CLOSE UP AT NIGHT.

New Zealand Graphic, Volume V, Issue 40, 4 October 1890, Page 18

WHY PLANTS CLOSE UP AT NIGHT.

Perhaps some of the youthful readers of the New Zealand Graphic have often wondered why plants go to sleep at night, and why some of them close up when it is going to rain. I will try in a few words to explain this very interesting little plan of theirs. A very clever man, Mr Darwin, wanted to find this out, so he w'atched, long ami carefully, the different plants, until he discoveied that the sap which is always moving through the tiny vessels of the plant stretches them whenever they will yield, and secondly, he found that warmth makes the surface of the leaves expand and be elastic. Take, for instance, the tulip Hower. This has closed when the sun left it, so all through the night it has been warm, and the soft, elastic inner surface of the petals is ready to stretch and yield, while the skin of the outer surface, which lias been chilled and stiffened during the night, yields much less readily. Therefore the inside petalswill push the outer ones open as soon as the air becomes warm. A chill coming on suddenly in the day-time causes the outer petals to contract and close. Thus when it is going to rain the pimpernel and the Held convolvulus close, because the air always gets colder before a shower. But there is another reason for their closing. Inside each little plant there is honey ai.d pollen, and in the open plants this would soon be washed away, if they did not shut it in tightly. The violet has no need to close, as her head hangs down sufficiently to protect her honey. Plants are very curious, you see, and well worth studying.