PHYSIOLOGY 293 



sudden liberation of hygroscopic or tissue tensions. It has already 

 been mentioned that the capillitia of the Myxomycetes and the 

 elaters of the Liverworts serve for the dispersal of the spores. In 

 the case of the Box (Buxus), the smoothiiSeeds are forcibly discharged 

 by the contraction of the pericarp, lik'6 a bean pressed between the 

 fingers. The dry fruit of Hum crepitans bursts apart with a report 

 like that of a pistol, and is scattered in pieces far and wide. The 

 turgescence and elasticity of the cell-walls give rise to the tension 

 which results in the forcible discharge of the sporangia of Pilobolus, and 

 in the ejection of the ascospores of many Ascomycetes. The bursting 

 and rolling up of the segments of the seed-vessels of Impatiens, by 

 means of which the dispersal of the seeds is effected, are due to the 

 sudden release of tissue-tensions. Similarly, the fruits of Momordica 

 elaterium and Ecballium dehisce suddenly and eject the seeds with 

 considerable force. It is unnecessary to cite further examples ; those 

 already given may be sufficient to call attention to a few of the different 

 means made use of for the dispersal of the reproductive germs. 



Germination. — The dry condition of the seed and the cessation of 

 all vital activity render the resting germ extremely resistant to the 

 action of external influences, and capable of maintaining its vitality 

 during the course of its dissemination, until it is ultimately fixed 

 in the earth. In effecting their permanent lodgment in the soil, 

 seeds are aided by the various structural peculiarities oe their 

 surface (furrows, bristles, hairs, etc.). The fruits of the Geraniaceae 

 (Erodiwm, Fig. 200) and Gramineae (Stipa, Avena sterilis, and species of 

 Aristida) are enabled, by means of movements due to hygroscopic 

 torsion, to bury themselves in the ground. In the case of 

 Trifolium subterraneurn and Arachis hypogaea the same result is 

 accomplished by the geotropic growth of the fruit-stalks, while the 

 seed-capsules of Linaria cymbalaria are deposited in the crevices of 

 walls and cliffs by the negative heliotropic movements of the fruit- 

 stalks. Nuts, acorns, and seeds buried by squirrels or other animals 

 in the ground and forgotten, or for any reason not made use of, often 

 germinate. The seedlings of Mangrove trees, Ehizophora and 

 Bruguiera, exhibit a most peculiar manner of growth to ensure their 

 lodgment in the ground. The seed germinates in the fruit before 

 it is detached from the tree. When the radicle has attained a 

 considerable length, the young seedling, separating either from the 

 cotyledons or from the fruit-stalk, falls to the earth ; it then bores 

 into the mud and is thus enabled to commence its growth without 

 delay. Many seeds and fruits acquire a more or less voluminous 

 Mucilaginous sheath, which serves a double purpose. Quince seeds, 

 Flax seeds, seeds of the Plantain, of Crucifers, the fruits of Salvia 

 Horminurn, seed of Cuphea and Cobaea (in the mucilage cells of which 

 delicate thickening bands are rolled up), afford the best-known examples 

 of such slimy envelopes, which, in addition to fixing the seed to the 



