PLANT PHYSIOLOGY. 113 



plants, of conidia among fungi, and of brood-cells and 

 brood-masses (gemmse) among liverworts and mosses. The 

 case is essentially the same where true buds, and even 

 branches separate from the parent plant, as the "bulblets" 

 in the axils of the leaves of some lilies, and in the inflo- 

 rescences of some onions, the runners of strawberries, the 

 trailing runner-like stems of buffalo-grass, the tubers of 

 many plants, as the potato, and perhaps the spontaneously- 

 deciduous twigs of cottonwoods and some willows. In all 

 these cases the essential feature is the separation from the 

 parent plant of one or more living cells, which continue to 

 grow, eventually producing a plant like the parent. We 

 go but a step further when we purposely cut off portions of 

 plants, which are then grown as " cuttings " by being 

 placed in moist earth. Even in the familiar operations of 

 grafting and budding, where the severed part is grown in 

 the tissues of another plant, the operation is essentially one 

 of asexual reproduction. 



196. Sexual Reproduction. — In marked contrast to the 

 foregoing are the various modifications of the sexual repro- 

 ductive process in which the essential feature is the union 

 of two cells (gametes) in the formation of the first cell of 

 the new plant. In the simplest cases two apparently sim- 

 ilar cells fuse into one, but as we pass to higher plants 

 there is an increasing difference between the cells con- 

 cerned. Moreover, while in the simpler cases the fusion 

 appears to involve the whole of each cell, in the higher 

 plants it is confined to the nuclei. 



197. Of Isogamy and Heterogamy. — Upon a close exam- 

 ination of sexual reproduction we find that in the classes 

 Ghlorophyceae and PhaeophycesB (see Chapter VIII), the 

 gametes may be' alike in size and other obvious characters 



