INFLTJENCE OF POLLEN. 139 



■well as forest trees, may in part be due to exhaustion in 

 bearing a full crop, and the necessity for a season or two 

 of rest and recuperation, is probably true, as some have 

 claimed ; but that it is largely due to general periodic 

 sexual excitement can scarcely be questioned, for similar 

 phenomena are of frequent occurrence in the Animal as 

 well as in the Vegetable Kingdom. 

 , Purposes oe Ceoss-Peetilizing. — The results sought 

 * in cross-fertilizing of yarieties, or the hybridizing of 

 species, are various, but the principal one is to produce 

 something different from either parent. Sometimes we 

 may aim to increase the size, or change the color, texture, 

 flavor or other characteristic of a fruit, or the size, form 

 and color of the flower or habit of a plant. Adaptation 

 of the various species and varieties of cultivated plants to 

 specific conditions is another, and often a very important, 

 object sought in producing cross-bred plants. There are 

 many species, and occasionally varieties, which have been 

 so closely and continuously inbred in their native habi- 

 tats or elsewhere that they have acquired a fixedness of 

 character that removals to other localities, and subjection 

 to widely different conditions fail to effect any material 

 change in their offspring ; but by crossing, and the intro- 

 duction of new sexual or other elements, the foundation 

 of generations, as it may be termed, is broken up, and 

 wider deviations from normal types soon follow. It may 

 sometimes be necessary to introduce an undesirable ele- 

 ment, in order to force a plant to break away from its 

 typical foi'm, but when we have succeeded in this, it will 

 not be difficult to breed out the undesirable characteristics 

 or properties. Then, again, we cannot know in advance 

 what wiU be the result of crossing any two plants of the 

 same genus or species, for even the mingling of two in- 

 ferior elements may result in the production of one supe- 

 rior to either of the originals ; still, we would not advise 

 using inferior materials in preference to superioij except 



