374 



BOTANY. 



cbyma, or fibrous tissue), the stems are frequently very hard. The 

 fundamental tissues frequently develop a good deal of mucilaginous or 

 slimy matter. 



Both stems and roots develop from a three-sided apical cell. The 

 apical cell of the root continually undergoes fission not only parallel to 

 its sides, but also parallel to its base i.e., at right angles to the axis of 

 the root. ' The daughter-cells thus cut off (k, k, Fig. 257a) constitute the 

 root-cap (pileorhiza) with which each root-tip is covered. 



The leaves, which unfold circinately, are often very large, and in 

 most cases are more or less lobed and divided, frequently becoming 

 several times compound. Their development is slow, the rudiment of 

 the petiole forming one year, and that of the blade the next, while the 

 opening or unfolding does not take place till the following year. The 

 growth is sometimes periodic, as in Qleichenia and Lygodium. In the 



FIQ. 268. 



FIG. 259. 



FIG. 260. 



Fig. 258. Under side of a fertile leaflet of Aspidium Filix-mas, with eight sori. 

 i, the indusium. Magnified. After Sachs. 



Fig. 259. A leaflet of Asplenium, showing the elongated sori, each covered by a 

 laterally placed indusium. From Le Maout and Decaisne. 



Fig. 260. A leaflet of Adiantum, showing the sori covered by indusia formed by 

 reflexions of the margin of the leaflet. From Le Maout and Decaisne. 



latter the leaf eventually becomes greatly elongated, resembling a 

 climbing stem. 



The sporangia are usually formed in clusters (sori) on the veins, on 

 the under side of the leaves, or upon their margins. The sori may 

 be distinct and rounded or more or less elongated, or they may be 

 confluent over considerable portions of the surface. In some cases 

 the sori are naked (as in Fig. 2576), but quite frequently each one 

 is covered by a cellular outgrowth of the leaf, called the indusium 

 (Figs. 258, 259, 260). In some cases the indusium is shield-shaped, its 

 short pedicel arising in the midst of the sporangia (Figs. 258 and 261) ; 

 in others it is more or less elongated, and attached by one of its edges 

 to the side of the sorus (Fig. 259) ; in still others a portion of the mar- 

 gin of the leaf is reflexed in such a way as to form the covering (Fig. 260). 

 Many other forms are common, and are to be found described in system- 

 atic treatises. The sporangia are more or less rounded bodies, usually 



