ACROGENOUS OR ACOTYLEDONOUS STEM. 



57 



104. The acrogenous stem in the young state is solid, but it fre- 

 quently becomes hollow in the progress of growth, by the rupture and 

 absorption of the walls of the cells in the centre. The bases of the 

 leaves remain long attached, but ultimately fall off, leaving marked 

 scars which are at first close together, but often separate afterwards 

 by interstitial growth. On these scars or cicatrices (cicatrix, a wound), 

 the markings of the vessels are easily seen, arranged in the same man- 

 ner as those of the stem with which they are continuous. The vascu- 

 lar system of ferns consists chiefly of scalariform vessels (fig. 62), 

 mixed with annular (fig. 57), and porous vessels (fig. 98 ter). There 

 are no true trachea with fibres which can be unrolled. In the stems 

 of Lycopodiaceae, closed tracheae or ducts (If 34) occur; and in Equi- 

 setaceae, the rings of the annular vessels are closely united. 



105. The stem of Ferns is generally of small diameter ; it does not 

 increase much laterally, after having been once formed, and it does not 

 produce lateral buds. Sometimes it divides into 



two (fig. 118), by the formation of two buds at its 

 growing point. This, however, is an actual divi- 

 sion of the stem itself, and differs from the branch- 

 ing of Exogenous and Endogenous stems. In the 

 Ferns of this country, the stems usually creep 

 along and under the ground, and the leaves which 

 they produce die annually without giving origin 

 to an .elevated trunk. In the common Brake 

 (Pteris aquilina), the arrangement of the vascular 

 system may be seen by making a transverse section 

 of the under-ground stem. The plant has received 

 its name aquilina, from a supposed resemblance to 

 a spread eagle, presented by the vessels when thus 

 cut across. 



106. In some Thallogens, which have been noticed as being stem- 

 less, the thallus or frond is supported by a stalk, in which there are 

 concentric circles, with divisions in the form of rays, and a sort of 

 pith. These are all forms of cellular tissue, however, without any 

 woody fibres. These appearances are presented by some large antarc- 

 tic sea- weeds, species of D'Urvillea, and by some lichens, as Usnea. 



107. There are thus three kinds of stems in the vegetable king- 

 dom: 



1. Exogenous or Dicotyledonous, having a separable bark; distinct 

 concentric circles, composed of progressive indefinite vascular bundles, 

 increasing at their periphery, the solidity diminishing from the centre 

 towards the circumference ; pith, enclosed in a longitudinal canal or 



Fig. 118. Vertical section of part of the forked stem or stipe of Alsophila perrotetiana. m, 

 Cellular central portion, z I, z I, Vascular zone, consisting of woody fibres and scalarifonn 

 vessels. The forking is caused by an actual division of the stipe. 



