530 MKASURES FOR PROTECTING GROWING PLANTS KROM LOSS OF HEAT. 



In iiifiuy instances the flower-buds and young flowci'S only assume an inverted 

 position periodically, i.e. only when a cold night is to be expected. Many umbel- 

 liferous plants are particularly noticeable in this respect, especially Falcuria 

 Rivini and the Burnet Saxifrage {e.g. Pimpinella Tnagna, and saxifraga) and 

 Carrot {e.g. Daucus Carota and viaximus). The sun has scarcely set when in all 

 these species the stalks which bear young flower-umbels bend downwards, crook- 

 like, so that the flower-buds, which during the day have been turned towards the 

 .sun, now face the earth, and the finely-divided involucral leaves spread out like an 

 umbrella over the nodding umbel. These finely-divided coverings radiate out 

 heat in the night without injury; the flower-buds below them, on the other hand, 

 are protected in the manner described against the nocturnal radiation so fatal to 

 them; whilst the heat they absorb during the day is thus in great measure, if not 

 entirely, retained. With the next sunrise the young umbels rapidly become erect; 

 their bent stalks rise up stiffly; and the flowei'-buds are again exposed to the 

 sun, as may be seen in the illustration of the Common Carrot {Daucus Carota) 

 inserted opposite (Fig. 132 ^>^). Later, when fertilization has taken place, and the 

 young fruits are developing, the necessity for protecting the stamens and pistils 

 from radiation no longer exists, and the periodic bending down of the umbel is 

 •discontinued. Young flower-heads of several scabiouses {e.g. tScabiosa lucida 

 and Columbaria) behave like the umbelliferous plants named, as also do the single 

 flowers of pansies {Viola tricolor), represented in fig. 132 ^' ^' in day and night 

 position next the umbels of the carrot. In numerous Compositse, Labiatse, and 

 plantains {e.g. Leontodon hastile, Mentha sylvestris, Plantago media, recurvata and 

 ■maritim,a) there are no such regular periodic movements; in these the capitula and 

 spikes are always pendulous while the flowei's are still in bud, and they remain in 

 this position as long as it is advantageous to them. Afterwards, when the nocturnal 

 loss of heat can no longer be injurious to the anthers and stigmas, or if other protec- 

 tive measures have been developed meanwhile, the axis of the inflorescence becomes 

 .stiffly erect. In many Compositse the involucres of the capitula or the peripheral 

 ligulate florets, and in other families the sepals and petals, bend up after sunset 

 over the stamens and pistils. They thus form a protecting roof under which 

 the temperature of the air alters comparatively slowly, and the delicate anthers 

 and stigmas are secured from i-adiation. 



A very striking contrivance for protecting against loss of heat by nocturnal 

 radiation is also observed in the seedlings of flowering plants, in those which 

 possess two seed-leaves or cotyledons. As long as the embryo surrounded by 

 protecting coats remains quiescent in the seed, the two seed-leaves are situated 

 with their upper surfaces in contact; later, when germination has taken place, 

 when the radicle has penetrated into the earth and the seed-coat is thrown oflf, 

 the two seed-leaves become separated, turn their upper sides towards the sky, so 

 that the seedling above-ground resembles an open book. In this position the 

 broad surfaces are exposed to the sun's rajs; they are also illuminated and warmed 

 as much as possible, and if they are coloured green, the formation of organic 



