140 



THE TREE FELJST. 



There are no sporangia in that division of Ferns known as the adders' tongues ; hut 

 the whole leaf is rolled up on either side of the midrib^ and becomes a containing organ* 

 At maturity the leaf opens by transverse valves, and emits the spores (Fig. 268). 



The foot-stalk of the frond is called the stipes, and consists of bundles or plates of 

 hard woody fibre and scalariform vessels, connected together by cellular tissue, which 

 pass down into the stem within the bark, and appear to form a part of the zones of wood. 

 The arrangement of the parts in the stein of the Tree Fern is very peculiar ; and 

 although it has no close resemblance to either the exogenous or the endogenous arrange- 

 ment, it seems to be more closely allied to the latter. Thus the rind or bark consists 

 of one or two layers only of cellular tissue, and is marked by the cicatrices of leaves or 

 fronds, arranged somewhat irregularly, and at considerable distances below, but 

 regularly and closely near the apex of the tree, showing that its leaves are produced at 

 the head only, and in successive clusters. Again, a large portion of the transverse 

 section of the trunk is seen to consist of cellular tissue ; and through this the wood 



passes. The points of resem- 

 blance to exogens are, that its 

 centre is occupied by a mass 

 of scalariform (Fig. 71) and 

 large spiral vessels, which in 

 some degree may represent 

 the medullary sheath ; and 

 the wood is arranged in cir- 

 cles, but only near to the bark, 

 and the circles have a wavy 

 outline. These pass up into 

 the fronds, or rather are sent 

 down from the fronds; and 

 as the fronds surround the 

 stem, the bundles sent down 

 from them lie side by side, 

 until they form a circle. 

 There are, moreover, lines of 

 communication between the 

 medullary cellular tissue and 

 the bark, which are the ana- 

 logues of the medullary rays. 

 There is a peculiarity in 

 the growth of the Tree Fern 

 viz., that the interval be- 

 tween the cicatrices enlarges 

 as the size of the tree in- 

 creases, showing that the 

 stem of the tree increases in height, not only at the apex for the time being, but after- 

 wards in the body of the trunk (Fig. 270). 



^ As there is no definite growing point in the sporule, its germination must differ 

 widely from the exogenous and endogenous forms of plants. The sporule, after extrusion 

 from the sporangia, bursts its envelope, and emits a leafy expansion from its centre* 

 which sub?equently forms a bud, and from thence a plant. 



Fig. 270. Tree Fern, forty feet high, growing in the moist 

 climates of small tropical islands. 



