ORGAJfS OF NUTRITION\ 91 



The best means of ascertaining the age of Palms is by noting 

 their increase in height in any one years growth, and then as 

 such stems grow almost uniformly in successive years, by know- 

 ing their height we can determine their age. This mode, however, 

 of calculating their age is very liable to error, and can be more- 

 over but of limited application from the absence of data to work 

 upon ; hence we must come to the conclusion that at present at 

 least, "we possess no certain means of determining the age of 

 endogenous stems. 



C. AcROGExors OR AcoTYLEDOxoTJS Stem. — The simplest 

 form of stem presented by Acotyledonous Plants is that of Mosses 

 {figs. 8 and 9) and Liverworts. In such a stem we have no true 

 vessels, but the whole is composed of ordinary parenchyma, 

 with occasionally a central cord of liber-cells. In the stems 

 of Club-mosses {Lycopodiacece), Pepperworts (Marsileacecg), and 

 Horsetails {Equisetacece), we have the simplest forms of acro- 

 genous stems which contain the peculiar vascular bundles 

 {Simultaneous), which are their especial characteristics. The 

 composition of these vascular bundles and their mode of 

 growth have been already described. (See page 67.) The vessels 

 found in the vascular bundles of the Lycopodiacese are sprral, 



f' and in those of the Equisetacese anmdar. All Acotyledonous 

 stems grow by additions to their apex, and hence the term 



^^' Acrogenov.s or summit growers, which is also applied to them. 

 In the Ferns (Filices), we have the Acrogenous stem in the 

 highest degree of development. Those which are indigenous 

 to this country are but insignificant specimens of such plants, 

 for in them the stem merely runs along the surface of the ground, 

 or burrows beneath it, sending up its leaves, 

 ov fronds as they are commonly called, into 

 the air, which die down yearly {fig. 12). 

 In warm regions, and more especially in the 

 tropics, we find such plants in the highest 

 degree of development. Here the stem, 



. — ^lled the caudcx or stipe, rises into the air 



I to the height of fifty or sixty feet or more 

 {fig. 13), bearing on its summit a tuft of 

 foliage. In their general appearance ex- 

 ternally these Tree-Ferns have great resem- 

 blance to Monocotyledonous trees, not only 

 in bearing their foliage like them at their 

 summits, but also in producing no lateral 

 branches, and being of uniform diameter ^Ma/lFenf' marked ex- 

 from near their base to their summits. The ternaiiy by'rhomboidai 

 outside of the stem of a Fern is marked dark-coioured 'projec- 

 with a number of scars, which have a more tioa^, c. 

 or less rhomboidal outline {fig. 181). The surface of these 

 scars present little hardened projections, c, or darker-coloured 



