CH. IVl 



SYMMETRY OF THE SHOOT 



67 



be. For instance, the young plant of Polypodium vulgare is from the first 

 prone, but the adult rhizome assumes its creeping position not by continuous 

 growth in the original direction, but by arching backwards over the pro- 

 thallus in a strong hyponastic curve (Fig. 39). Again, in the case oi Polypodiwn 

 heracleum the dorsiventral, climbing rhizome is normally appressed to the 

 support. But if it is grown on soil it takes a spiral curve, owing to stronger 

 growth of the leaf-bearing side (von Goebel, Organographies 19 13, P- 221, 

 Fig. 2 1 6). Such cases show that though gravity may have been a determining 

 influence causing the dorsiventrality of the shoot in the first instance, it does 

 not always account directly for the positions which such dorsiventral shoots 

 may assume. 



Fig. 57 bis. Polypodium vulgare, dorsiventral rhizome with two rows of alter- 

 nating leaves. Those of the preceding year represented by scars ; those of the 

 current season have their petioles attached. A lateral branch of dichotomous 

 origin shows the character of the dichopodial system. (After Velenovsky.) 



The arrangement of the leaves on the axis in Ferns is alternate, a 

 condition initiated by the first seedling leaf, which is always solitary, and 

 may be followed by leaves arranged on 

 a spiral plan. In creeping Ferns the 

 leaves are usually inserted alternately 

 right and left, forming two lax ortho- 

 stichies upon the dorsiventral rhizome. 

 This is well seen in Polypodium vulgare 

 (Fig. 57 bis), and in the Bracken. But in 

 certain cases the leaf-bases may appear 

 as though forming a single row, owing 

 to the close approximation of the two 

 orthostichies on the upper surface of the 

 rhizome, as in Lygodium (Fig. 58). In 

 ascending or upright stems, on the other 

 hand, the arrangement of the leaves is 

 spiral, with divergences which approxi- 

 mate to those shown by Flowering Plants. ^xg. -,8. The dorsiventral and dichotomous 

 A good instance is seen in Ankyropteris rhizome oi Lygodium scatidetis, with leaves 



„ . ^ ., „ - , T-, . • apparently in a single row. (After Velen- 



(jrrayz, a lossil rern irom the ralaeozoic ovsky.) 



