42 THE PHENOMENON OF 



sive opening of the flowers, which is especially important 

 in the case of annuals, — or in a succeeding period of 

 vegetation. The latter condition is particularly important 

 for such plants, as, in consequence of the contrivance of 

 the organs of fertilisation, rarely bear fertile seed, as is 

 the case with most of the Orchideae, — as also for such as 

 through their situation are often prevented from flowering 

 for a long time, as is the case with many water-plants, 

 when the level of water remains long very high. Thus 

 Littorella lacustris, which never flowers under water, 

 maintains and increases itself by lateral runners, year 

 after year, at the bottom of the lakes of the Black Forest, 

 and only comes into flower when the water retreats, in 

 the driest years, which scarcely recur oftener than once 

 in ten. Similar conditions are exhibited by many nemoral 

 plants, as for mstance, the woodruff'; when the shade of 

 the wood is too dense, or even when too free an opening 

 of the wood interferes with its flourishing above-ground, it 

 maintains itself many years without flowering by subter- 

 ranean runners, waiting from generation to generation 

 the return of a season favorable to its success. The well- 

 known phenomenon that annual plants almost entirely 

 disappear in the extreme north and in the Alps, likewise 

 deserves to be mentioned here, since it shows how, in 

 proportion as the ripening of seed is endangered by cold, 

 a formation of sprouts adapted to the persistence through 

 the cold season takes place. Lastly, the formation of 

 sprouts is of especial importance for hybrid plants, which 

 as a rule can only be maintained and increased by 

 naturally or artificially detached sprouts. The frequent 

 experience that hybrids of annual or biennial plants, e.g. 

 the hybrids of the genus Verbasciim* acquire a duration of 

 many years through continued formation of sprouts from 

 the old " stock," is a speaking testimony of the inter- 

 position of sprout-formation in cases where propagation 

 by seed is difficult or impossible, since in this instance 



" I havf observed tliis plicnomoiioii also in Uic hybrid between lEiwihem 

 hkiiMs aird MUjicata., not unl'rcr]iicu( m tlie district of Freiburg. 



