IRRIT ABILITY. 127 



form of large succulent roots, which is to be used when the plant 

 requires large supplies of nourishment in the perfection of its 

 seed. But plants which do not so obviously provide this accu- 

 mulation of food, nevertheless, require a fit state of development 

 before they can perfect their fruit. The gardener is well ac- 

 quainted with this fact, since he knows that Melons and like 

 fruits, which set early, either uniformly fall off, or are diminutive 

 and useless ; but if they are not permitted to set till the vine is 

 well developed and filled with sap, they then grow rapidly and 

 come to perfection, having a full supply of food laid in store for 

 their use. It is a well-known fact, also, that when a fruit-treo 

 is prevented from bearing one year, the fruit for the next 

 year is much better than the ordinary fruit of the tree ; the tree 

 having accumulated food during the year of rest, which con- 

 tributes to the abundance and perfection of the fruit. Trees 

 also sometimes cease to bear only every other year, either from 

 age or from want of sufficient nourishment in the soil in which 

 they grow ; they cannot bear the exhaustion attendant on the 

 perfection of a yearly crop of fruit. 



234. The fleshy receptacles also of many plants afford nour 

 ishment during the perfection of the seed. In some of the 

 grasses, when they grow in moist soils, they become tuberous, 

 laying up food in the tubers for times of drought. 



Section 7. — Irritability. 



235. The vitality of plants is often exhibited by various 

 spontaneous motions ; by the sensible effects produced by the 

 actions of external agents ; all of which phenomena are attribu- 

 ted to irritability. 



Of the former of these phenomena the most common is what 

 is generally called the sleep of plants. In plants with compound 

 leaves the leaflets often close on the approach of darkness, and 

 expand again on the return of day. Many flowers also undergo 

 the same changes. Some flowers, however, are unable to sustain 

 the light for the whole day, and close their flowers under the di- 

 rect rays of the sun. In some cases, also, the calyx and floral 

 leaves embrace the flower, seemingly for the purpose of protect- 

 ing it from the action of the cold and moisture of the night. 

 Most of the preceding phenomena are, no doubt, due to the 

 action of the light, since they may be made to take place by 



How with early fruits? With fruit trees?— 234. What is said of the 

 roots of some grasses ? — 235. What is meant by irritability ? What of the 

 sleep of plants ? 



