530 MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



In many instances the flower-buds and young flowers 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 Falcaria 

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

 Carrot (e.g. Daucus Garota and Tnaxiinus). 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 stiflly; and the flower- buds are again exposed to the 

 sun, as may be seen in the illustration of the Common Carrot (Daucus Garota) 

 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. Scabiosa 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, Labiatae, and 

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

 maritima) there are no such regular periodic movements; in these the capitula and 

 spikes are always pendulous while the flowers 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 famihes 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 radiation. 



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 ofl, 

 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 rays; they are also illuminated and warmed 

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



