STRUCTURE OF THE STEM. 91 



there are a great many intermediate conditions. In flowering- 

 plants which only live for one year, and are hence known as 

 annuals, the stem is usually herbaceous ; that is, it consists 

 almost entirely of soft cellular tissue, but contains, however, 

 some bundles of woody fibre and vessels, which may be traced 

 to the stalks of the leaves. These are commonly known as the 

 strings of vegetables whose stalks or roots are eaten, such as 

 Asparagus or Turnips ; and that degree of stringiness which 

 makes such plants unfit for the table, when their growth is too 

 far advanced, results from the increased formation of woody fibre, 

 which takes place towards the end of the season. In those plants, 

 however, the duration of whose life is greater, the stem gradually 

 becomes consolidated by the formation, in each year, of a new 

 set of these bundles ; so that, in time, the soft cellular part bears 

 but a small proportion to the whole. Herbaceous and woody 

 stems, however, are at first formed upon the same plan ; the 

 structure of a first-year's branch of an Oak, for example, being 

 essentially the same with that of the stem of an annual Pea. 

 The foundation is laid, as it were, by an extension of the cellular 

 tissue from which it springs at first, this being the kind of struc- 

 ture which most rapidly increases ; and the consolidation of this 

 is then gradually effected, by the deposition of woody fibre in 

 its substance. 



125. But the stems of Flowering-plants are not all formed 

 upon the same plan ; in fact there are two different and nearly 

 opposite modes, in which the woody bundles are arranged in the 

 stem ; and to one of these, they may be all referred. If the 

 stem of the Asparagus be cut across, it will be seen that they 

 are distributed at intervals through its whole substance. On 

 the other hand, in the stem of a Pea they would be seen regu- 

 larly arranged in a circle, a little beneath the exterior. The 

 former, if it continued to live and grow, would have its new 

 bundles deposited in the interior of the stem ; whilst the latter, 

 if it survived a second summer, would have a new circle of woody 

 bundles deposited on the exterior of the first. The former is then 

 called an ENDOGEN, (growing from within) ; the latter an EXOGEN 

 (growing outside). To the first division belong the Palms, 



