COURSE OF THE BUNDLES IN THE STEM. 



^95 



st>\ 



Sect. 87. While in the last-named cases accessory medullary bundles occur, and 

 in many Cyatheaceae accessory medullary and cortical bundles in addition to a 

 typical tube of bundles, there is found in Pleris aquilina and Polyhohya Meyeriana a 

 bundle-tube constructed as in the type with a strong upper bundle, and this is 

 strengthened by a much divided cortical system of bundles. 



In the seedling of Pteris aquilina^, till the development of the seventh or ninth 

 leaf, there is one axile bundle, which traverses the stem starting from the point of 

 union of the first leaf with the first root : in transverse section it is deeply grooved, 

 and half-moon-shaped : bundles pass from it into the leaves ; after the formation of 

 the seventh to ninth leaf ' the stem forks.' Both branches of the fork increase rapidly 

 and considerably in thickness, and the course of the 

 vascular bundles in them is altered. The lateral 

 opening of the axile bundle is widened, then the 

 upper half is separated from the lower ; there are 

 now two bundles, an upper and a lower one, which 

 split now and again into thinner branches, while 

 these are soon again united. When the branches 

 of the fork have attained the length of about 6cm, 

 and a- thickness of about 4"im^ weaker branches 

 come off from both the bundles : these run near 

 the surface (in the cortex), and here form a peri- 

 pheral network with long and narrow meshes, in 

 which the upper central bundle is distinguished 

 from the rest by its greater width. This structure 

 is retained by the mature rhizome (Fig. 143): the 

 number of the peripheral bundles rises to twelve 

 in the transverse section. Two thick brown plates 

 of sclerenchymatous fibres lie between the inner and 

 outer systems of bundles, and are only separated 

 from one another at the two sides of the stem by 

 a narrow slit filled with parenchyma : they are often 

 joined at one side, often even all round so as to form a closed tube. Branch- 

 bundles from both nets of bundles pass into the leaves and branches: roots arise 

 from the outer ones only. At those points of exit, as also at the base of the petiole, 

 the two nets anastomose by single transverse bundles. Throughout the whole of the 

 rest of their course they are without connection with one another in many weak 

 specimens ; in strong rhizomes, ccording to Stenzcl, thin connecting bundles pass 

 from the margins of the inner ' undies to the lateral outer bundles, while the upper 

 and lower bundles of both systems are connected by single short branches, which 

 pass through holes in the bands of sclerenchyma. 



In the main axis of Polybotrya Meyeriana'^ an inner network is found sur- 

 rounding a narrow pith, and composed of 3-7 strong bundles arranged in a 

 circle when seen in transverse section; it corresponds in composition to that above 



I-IG. 143.— I'teris aquilina; stem (rhizome); 

 slightly enlarged. A transverse section, r 

 brown sclerenchymatous layer of outer cortex, 

 / colourless soft parenchyma, ig vascular 

 bundles of the inner zone, ag the broad upper 

 bundle of the outer zone, pr the plates of 

 brown sclerenchyma separating the two zones. 

 B the upper main bundle of the outer zone of 

 the stem (s(\ and its branches (st' , si"), with 

 the branch bundles which pass into a leaf (*) 

 prepared free ; it — ii outline of the stem. 



' Hofmeister, I.e. p. 620.— Metleiiius, Angiopleris, p. 561. — Stenzel, /. , 

 '' M ettenius, /. r. p. 559, Taf. VII. 



