The Orderly Cycles Pursued in Growth 359 



responding to the stimulus of light, they open out, turn green, 

 and serve as the first foliage. At least such is the procedure in the 

 most typical cases, though there are many variations in detail, 

 including especially a great many cases in which the cotyledons 

 remain in the seed, sending up only the plmnule. Meanwhile 

 this embryo has continued to absorb water, with which its cells 

 have swelled greatly; and its stored food has been largely con- 

 verted to new wall and hving substance. Finally it stands up 

 stiffly, many times larger than at first, but with no new parts and 

 even less of solid substance. It is now all ready to begin its in- 

 dependent life. 



Thus is the new plant born. 



3. The Growth Cycle: the Seedling. — The stages of develop- 

 ment, while distinct in the main, overlap in some places. Thus 

 the root develops considerably in germination, and early in this 

 stage begins to branch. Its new cells are all formed at a definite 

 growing point, whence they radiate in regular lines backward, in- 

 creasing in size, as shown very clearly in our earlier figure 53, and 

 diagrammatically in a later figure, 139 D. The branches of roots 

 start always from the fibrovascular bundles and have therefore 

 to break or dissolve their way out through the cortex (figure 67), 

 a method which seems clumsy, but is doubtless the best that the 

 plant can do. Their places of origin are fixed largely by exter- 

 nal stmiuli, — the contact of greater warmth, moisture, aeration, 

 mineral supply and the like. The guiding stimulus of their sub- 

 sequent growth is gravitation, which sends them radiately out- 

 ward in directions of least interference with one another, though 

 they, and especially the later branches, are swimg from the 

 geotropic angles, and given their final details of position, by ad- 

 vantageous responses to various minor stimuli. Thereafter, so 

 long as the plant lives, these roots grow, branch, and are guided 

 continuously in this manner. Meantime the embryonic cells be- 

 tween the cotyledons become active and push up a cone of cells 

 which constitutes the first bud. As this bud becomes larger the 



