MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 529 



of heat which are not at all connected with transpiration, or only to a slight 

 degree, are brought together here in the form of a general sketch. 



First of all in this respect are to be considered flowers of comparatively rapid 

 growth, whose parts therefore require much warmth, but for which many of the con- 

 trivances suited to foliage-leaves are not well adapted as protective measures against 

 loss of heat, since other functions might be encroached upon. And yet these flowers 

 especially require an abundant protection against loss of heat, even more than 

 other plants on account of their great sensitiveness. If in the spring a blossoming 

 snowdrop, having already penetrated the soil, is surprised by a frost, the flower-stalk 

 and the leaves sink down as if withered, while the flowers outwardly are not 

 at aU altered. Anyone observing this might think that the green stem and leaves 

 had been injured, but that the flowers, on the contrary, had survived the catas- 

 trophe without harm. But exactly the opposite is the case. The stem and leaves 

 become erect with returning warmth and continue to grow, but the pollen in the 

 anthers of the flower is dead; also the ovules, styles, and stigmas are aflected so 

 that they wither and shrivel up: obviously the production of ripe seeds is then out 

 of the question. It is also observed that the pollen in the anthers is best formed 

 when the flower-buds in question are warmed through by the sun, and when the 

 blossoming plants grow on a free open space which the sun's rays can reach. 

 Moreover, the floral envelopes develop much better in such spots than in cool shady 

 places; they become larger, exhibit brighter colours, and consequently are more often 

 visited by insects than those which receive relatively little light and heat. But 

 the danger that the flowers and flower-buds will again lose by radiation, through 

 the night, the heat which they have gained during the day is most likely to be 

 felt in open, unshaded habitats, that in consequence of the great loss of heat 

 the formation of the poUen in the yet closed anthers will be injured, and finally, 

 that the petals will also be disturbed in their growth and function. In order to 

 avoid this, in many cases the flower-buds and also the open flowers are pendulous, 

 beU-shaped, and tubular, or leaves become arched in the shape of a dome, cap, 

 or umbrella above the stamens and pistils, in which case the inner portions of 

 such flowers are hidden as in a niche or groove. In these hidden nooks they are 

 comparatively well -protected against loss of heat, and at least no radiation of 

 warmth towards the night sky proceeds from the anthers and stigmas. Only the 

 coverings spread over the stamens and pistil, as a protecting roof, lose during the 

 night a great part of the heat obtained in the day. These, however, are not so 

 much endangered, since they have already obtained their normal size and have no 

 need of heat for further growth; besides, they are usually clothed with air-contain- 

 ing, hairy structures, surrounded by dry membraneous edges or entirely transformed 

 into dry parchment or paper-like scales, in which case they can suffer no further 

 damage from loss of heat. The air in the pendulous bell-flowers is 1-2 degrees 

 warmer, in the morning before sunrise, than the surrounding air; here, closed in, 

 it remains practically unaltered during the night; and this of course is exceedingly 



useful to the warmth-loving anthers and stigmas there hidden. 



Vol. I. ^ 



