72 ACEOGENOUS OR ACOTYLEDONOUS STEM. 



tion 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 

 manner as those of the stem, with which they are continuous. The 

 vascular system of ferns consists chiefly of scalariform vessels (fig. 64), 

 mixed with annular (fig. 62), woody and pitted vessels (fig. 116 ter). 

 There are no true tracheae with fibres which can be unrolled. In the 

 stems of Lycopodiacese closed tracheae or ducts occur ; and in Equi- 

 setaceae the rings of the annular vessels are closely united. 



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. 137), by the formation of two buds at 

 its growing point. This, however, is an actual 

 division of the stem itself, and differs from the 

 usual branching 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, with- 

 out giving origin to a conspicuous trunk. In the 

 II % f /I common Brake (Pteris aquilina), the arrange- 



i I it:"' //' ment of the vascular system may be seen by 



{* — LS—i/) making a transverse section of the underground 



ii rn. ^'i stem. The plant has received its name aj-MiKsM, 



'^' ' from a supposed resemblance to a spread eagle, 



presented by the vessels when thus cut across. 



The axis of Lycopodiacese or Club-mosses (fig. 138) exhibits a 

 vascular bundle of scalariform vessels and closed spirals. The bundle 

 is developed in an upward direction as the stem grows, each inter- 

 node having its permanent bundle. Vessels pass from the stem to 

 the leaves. 



In Equiseta or Horse-tails (fig. 139) there is a circle of vascular 

 bundles towards the exterior of the aerial stem ; this vascular ring is 

 covered by cortical cells of difierent kinds. The Equiseta have 

 underground stems, from which the aerial branches are sent up 

 annually. In some species the aerial stem attains a height of 

 upwards of 30 feet. The largest species in Britain (Equisetum 

 maximum), may be seen 5 to 6 feet high, with a diameter of half- 

 an-inch. The aerial stem of the plant consists of hollow internodes, 

 each with a transverse diaphragm at the base, and a sheath at the 



Fig. 137. Vertical section of part of the forked stem or stipe of Alsophila perrotetiana. 

 m. Cellular central portion. zl,zl, Vascular zone, consisting chiefly of woody ftl)res and 

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



