GROWTH. 133 



by which means the stem becomes much harder at the 

 base than elsewhere. 



216. In the Grasses the stem is hollow, having a dia- 

 phragm or partition at the various nodi ; the vacuities 

 arise from the inability of the centre of the stem (ori- 

 ginally solid) to keep up with the increased growth, 

 and the consequent quick extension of the circumference 

 from it. 



217. Of the Acotyledonous stem, the only example 

 we shall choose will be that of an aborescent Fern. 

 The stems of these plants vary in appearance according 

 to the species, and to give a general view of them is 

 rather a difficult matter ; but they all show separate bun- 

 dles of woody matter like the Endogenous stem, and 

 these bundles are arranged in a circular manner near the 

 circumference of the axis. The bundles themselves may 

 be round, elliptical, irregular, or sinuous, and contain in 

 the centre spiral vessels, or modifications of reticulated 

 ducts. A cellular parenchymatous tissue generally occu- 

 pies the centre of the stem, which, when fresh contains 

 much fluid and large grains of amylaceous matter. The 

 external covering of the stem, or bark, if w r e may so 

 term it, according to Mohl, is composed of two layers, 

 which gradually run together so as to form only one. 

 The growth of these stems takes place by development 

 of matter from the terminal leaves, the wood being 

 formed of the converted bases of their petioles. 



218. Mohl traces a relationship between these stems 

 and those of Cycas, Treviranus with the Coniferae, and 

 Link and Meyen with the Endogense. 



GENERATION. 



219. The last function we have to speak of under the 

 physiology of plants, is at the present moment a com- 

 pletely unsettled question, in regard to many of its rela- 

 tionships, on account of the diversity of opinion enter- 



