544 PHYSIOLOGY. 



or less indefinite period (capable of being shortened or prolonged 

 within certain limits by external causes) in a state of rest (seeds, 

 resting -spores of Alyce, &c.), and then undergoes peculiar internal 

 changes before recommencing development (germination) in order 

 to grow up into a new plant. 



Seeds and resting-spores (and to a less extent the resting- organs pro- 

 duced in vegetative propagation, as bulbs, tubers, &c.) are organized in a 

 manner especially adapted to preserve the latent vitality from injury by 

 external influences. They can withstand great variations of heat or cold, 

 especially in the absence of moisture. Most seeds will bear a temperature 

 very far below freezing-point if kept dry. Wheat left in the Arctic 

 Kegions by the crew of the ' Polaris,' and brought home by Sir George 

 Nares, after two years, was found to have its germinating power unim- 

 paired, and many will even bear an exposure to 100 or 110 Fahr. in dry 

 sand. Prolonged immersion in water at 120 kills most seeds, unless the 

 skin is very thick and they contain oil instead of starch in the endosperm. 

 Some seeds will bear a short immersion in boiling water ( Veronica) ; but 

 the seeds of Cereals, Beans, Linseed, and other plants scarcely survive a 

 15 minutes' soaking in water of 110, while they will bear 140 in steam 

 and 170 in dry air. 



Some seeds naturally lose their vitality very soon : this is the case with 

 the seeds of Cqffea, Magnolia, &c. j while other instances are related in 

 which it has been preserved for centuries. The cases related of the 

 germination of Wheat taken from Egyptian Mummies are fallacious : 

 but well-authenticated instances exist of long preservation. 



The resting-spores of Oonfervoids (Protococcus) have been revived after 

 remaining for years in herbaria ; and it is in curious relation to their 

 growth in shallow pools, often dry in summer, that the resting-spores of 

 these plants appear to require to be dried before they will germinate. 

 Mr. Munby found a bulb of a species of Narcissus sprouting in his 

 herbarium after it had been gathered (in Algeria) upwards of twenty -two 

 years. This bulb, removed into the greenhouse and potted, produced 

 flowers. 



Periodicity of Growth. Plants are subject to a periodicity in 

 their vital phenomena, partly dependent on their own laws of 

 growth, partly on the seasons in the climate where they grow. As 

 dependent on special laws may be noted the differences between 

 annual, biennial, and perennial plants (properly so called), between 

 deciduous-leaved and evergreen trees, &c. 



Annual plants are such as germinate from seed, produce their whole 

 vegetable structure, flowers, fruit, and seed, and die away in one season, 

 between spring and autumn : such are the summer annuals of our gardens. 

 Biennials sprout from seed in one season, and bloom, bear fruit and seed, 

 and die in the second; the Turnip, Carrot, (Enothera Uennis, &c. are 

 examples of this. Perennial plants exhibit several varieties of condition. 

 Herbaceous perennials (like the Daisy, Primrose, Garden Flag, &c.) ger- 

 minate in one season, and produce a subterraneous rhizome, of indefinite 





