DEVELOPMENT OF THE OSTRICH FERN. 



31 



or sixth and probably until the stem reaches its pei'manent form, the apical cell under- 

 goes bifurcation so that at an early period they consist of two nearly equal lobes. Just 

 how far the subsequent branching of the leaf is due to dichotomy is difficult to deter- 

 mine, but probably in the later formed fronds of the young plant it does not continue 

 beyond the first division, the later divisions, as in the fronds of the mature plant, beino- 

 monopodial. 



A cross-section of the stem of a plant with three leaves completely developed showed 

 that the stem-bundle was elliptical in outline. The center was occupied by a mass of 

 elongated parenchyma cells lying outside of which was a mass of short spiral or reticu- 

 late tracheids. ISText followed two or three layers of elongated cells much like those oc- 

 cupying the center of the bundle, and surrounding the whole a bundle-sheath formed of 

 naiv'ow cells with brown walls. From the stem-bundle the bundles going to the leaves 

 and roots branched ofi", their buudle-sheaths being continuous and the tracheary tissue 

 connected. 



The stalks of the fii-st three leaves are provided with a single axial bundle; but in the 

 fourth, as in the mature leaf, there are two, one on each side of the deep groove in the 

 front of the stalk. These two bundles arc separate nearly or quite to the base, the stem- 

 bundle being deeply cleft at the point where they issue from it. They are nearly round 

 when seen in cross-section, the tracheary tissue being arranged in a band consisting some- 

 times of a single row of spiral ti'acheids, only four or five in luunber, the arrangement re- 

 calling that found in the older roots. The rest of the bundle is made up of elongated cells 

 with rather thick walls whi(;h appear to swell slightly in water. The whole is surrounded 

 by a sheath. 



The number of roots exceeds that of the leaves, there being six fully developed roots 

 in a plant inwhich the fourth leaf was not fully grown, besides several that were not 

 fully developed. 



Section IV. The Leaf. 



The leaves arise from single segments of the apical cell of the stem. The first indica- 

 tion of the young leaf is the elevation of the surface of the segment, the apical cell of the 

 leaf being formed very early from one of the surface cells. A wall is foi-med in this cell 

 inclined to its outer wall so as to meet one of the lateral walls. This is probabh^ folloAved 

 by a second wall incliued to the first, and meeting it so as to include a wedge-shaped 

 cell, the apical cell of the leaf. From this time the growth of the leaf is dependent upon 

 the further development of this apical cell, Avhich divides by walls directed alternately 

 right and left into two series of segments. The early growth of the apical cell is rapid, 

 soon resulting in a flattened cone with a bi'oad base, the side in contact with the stem 

 being more flattened than the outer face, which grows more rapidly, so that the young 

 frond soon begins to bend inward over the end of the stem. This inequality becomes 

 more and more strongly marked as growth proceeds, finally causing the frond to be 

 closely coiled as is well known in all ferns. 



The leaf-stalk develoj^JS at first much moi-e rapidly than the lamina, which remains very 

 small, pi'oportionately, until the latter part of the summer previous to its uufolding, when 

 it undergoes a great increase in size; the unfolding of the leaf being little more tliau an 



