ORGANIZATION OF THE STEM. 49 



plant perfect in all its parts and dependent for its future 

 growth and sustenance on its leaves and roots. 



Let us now examine the successive modifications of the inter- 

 nal structure, from the commencement of germination to the 

 growth -of the first pair of leaves and the completion of this 

 the first stage of vegetation. At first the embryo consists 

 wholly of _cellular tissue ; as soon, however, as it begins to 

 grow, even while the cotyledons only are developing, some of 

 the cells begin to elongate into tubes longitudinally, assuming 

 the form of vascular and woody fibre. These nascent wood-cells 

 extend upwards into the cotyledons, and downwards into the 

 radicle, and are finally seen in a cylindrical form in the centre 

 of the stem. This longitudinal elongation of the cells does not 

 take place at random, but certain determinate cells only 

 thus change their character, whilst the others wholly retain 

 or depart but slightly from their primitive form. A horizontal 

 section of the plumule at this stage of development will show 

 this. 



The sap elaborated in the first pair of leaves contributes to 

 the upward growth of the plumule or young stem, and to the 

 development of the second paif of leaves ; the new wood-cells 

 extend through them" to form their frame-work, making the 

 woody stratum in the second internode as it lengthens, and con- 

 tributing at the same time to the increase of the stem beneath 

 them; and the same process is repeated throughout the whole 

 growth of the season with every fresh development of leaves. 



The woody fibre having rapidly increased, ascending and 

 descending the stem with the growth of every new set of 

 leaves, the medullary rays ultimately become so much com- 

 pressed, that they assume the form of fine lines radiating from 

 the centre to the circumference of the stem. 



5* 



