FILICINEM. 



429 



especially striking, from their great numbers and from being frequently flat and leaf- 

 like ; the younger leaves are generally entirely covered and concealed by them. 



After these preliminary particulars, we may now turn to a consideration of the 

 mode of growth of the separate organs. 



The growing end of the stem sometimes far outruns the point of attachment of 

 the youngest leaves, and then appears naked, as in Polypodium vulgare, P. sporodo- 

 carpu?n, and other creeping Ferns, as well as in Pteris aquilina, where, according to 

 Hofmeister, it frequently attains in old plants a length of several inches without 

 bearing leaves. Mettenius states that in many Hymenophyllaceae leafless prolon- 

 gations of the axis of this kind have been taken for roots. In other cases, on the 

 contrary, especially in Ferns with an erect growth, the increase in length of the 

 stem is much slower, its apex remaining enclosed in a leaf-bud. The stem generally 

 ends in a flat apex ; sometimes, as in Pteris, it is even imbedded in a funnel-shaped 

 elevation of the older tissues (Fig. 301, E). The apex of the stem is always occupied 

 by a clearly distinguishable apical cell, which is either divided by walls alternately 

 inclined, and then resembles, when viewed from above, the transverse section of 

 a biconvex lens ; or it is a three-sided 

 pyramid, with a convex anterior surface 

 and three oblique lateral surfaces, which 

 intersect behind. The outlines of the 

 segments, which are in the first case in 

 two, in the second case in three rows, 

 or arranged with more complicated 

 divergences, soon disappear in conse- 

 quence of numerous cell- divisions and 

 of the displacement caused by the 

 growth of the masses of tissue and 

 leaf-stalks surrounding the apex. The 

 apical cell, for instance, of Pteris aqui- 

 lina^ is wedge-shaped, the segments 

 on the horizontal stem forming a right 



and a left row ; the edges of the apical cell face upwards and downwards 

 (Fig. 300). The same is also the case, according to Hofmeister, in Niphoholus 

 chinensis and rupestris, Polypodiiini aureum and punctulatum, and Platycerium 

 alcicorne. In Polypodi^vi vulgare he states that it is sometimes wedge-shaped, 

 sometimes pyramidal with three faces ; the last-named form occurs also in Aspidium 

 Filix-mas, &c. As a rule it may for the present be assumed that creeping stems 

 with a bilateral development have a wedge-shaped apical cell, upright or ascending 

 stems with radiating rosettes of leaves one that is a three-sided pyramid. 



The further relationships of the segments of the apical cell of the stem to the 

 origin of the leaves and to the building up of the tissue of the stem itself are 

 still but litde known in detail. It cannot be doubted that each leaf results from 

 a single segment only, and that this segment-cell is devoted from an early period 

 to the formadon of the leaf, but it appears doubtful whether the segments always 

 form leaves, and if not what number of sterile segments intervenes between those 

 from which a leaf is developed. 



Fig. 300.— Apical view of the end of the stem of Pteris aqui- 

 lina; y the apical cell of the stem ; x the apical cell of the youngest 

 leaf; h h hairs which cover the apical region surrounded by a 

 cushion of tissue. 



