DEVELOPMENT OF THE OSTRICH FERN. 35 



secondary racbis, the former the mesopbyll and epidermis, as well as the veins. Each 

 outer cell, divides into two by a wall parallel to tlic primary wall, and these in turn divide 

 farther into an inner and outer cell, as was the case with the original segment (PI. G, 

 fig. 7). A single row of marginal cells is thus formed about each lol)e of the pinna, 

 which continues to undergo the same divisions. The lobes of the pinna arise from it in 

 precisely the same vVay that the pinnae are formed from the frond. 



The cells lying immediately within the marginal cells divide by walls parallel to the 

 surface of the leaf, so that a flat mass of cells is formed of which the outer laj^ers are 

 prismatic, foi'ming the young epidermis, the inner ones polyhedral constituting' the meso- 

 pbyll. Up to this point the cells of the young lobes resemble each other, all containing 

 finely granular, colorless protoplasm and having a large nucleus. 



The cells of the young secondary racbis divide also by walls parallel to the surface of 

 the leaf so as to separate them into a central cylinder, which is transformed by longitu- 

 dinal walls into the procambium of the bundle, and the overlying ground-tissue and epi- 

 dermis. The central cells of the young lobes become also divided by a series of longi- 

 tudinal walls into a mass of narrow procambium cells running nearly to the tip of the 

 lobe. The lateral veins are formed in acropetal succession, but do not begin to form until 

 tlie lobe has reached some size. If the lobe has a definite apical cell, each lateral vein 

 j)robably corresponds to each segment cut off from it; but when there is no apical cell, 

 as is much more frequently the; case, it is impossible to trace them back to a single cell. 

 They arise from rows of cells in the young mesopbyll extending from the midr'^o to the 

 margin. These cells become divided by longitudinal walls so as to form the procambium 

 of the young vein. The end of the midrib sometimes forks, so that there here remains a 

 trace of the dichotomous branching of the veins of the earliest leaves. Cases Avhere 

 several veins appear to arise by dichotomy are sometimes met with, but these are excep- 

 tional, and probably never occur in fully developed fronds. 



The formation of the young epidermis, ground tissue and procambium, is completed 

 the year before the fronds unroll, and in the stalk the bundles develop a considerable 

 part of the tracheary tissue ; but it is not until the fronds begin to unfold that the differ- 

 ent tissues become fully developed : and after this begins the process is a very rapid one. 



The 3-oung ejndermal cells increase very much laterally but with little increase in 

 deiith, so that they become tabular witli extremely irregular sinuous outlines (PI. 6, 

 figs. 10-14) . The mesopbyll, which is at first composed of polyhedral cells with no inter- 

 cellular spaces, becomes very loose by the formation of intercellular spaces arising from 

 the unequal growth, and separation of the cells. The fibro-vascular biindles develop 

 their different tissues now, stomata are found in the epidermis of the lower surface of the 

 frond, the frond becoming in the meantime completely uncoiled, and the leaf is complete. 



The formation of a stoma is preceded by the cutting off of a preliminary cell (PI. 6, 

 fig.' 12), from which the mother-cell of the stoma is subsequently formed. This first wall 

 is V -shaped, or nearly semicircular, and is soon followed by a similar but smaller one 

 nearly parallel to it (fig. 10) . The cell cut off by this second wall is the mother-cell of 

 the stoma (fig. 10 y) . The protoplasm in tliis cell is more dense than in the surround- 

 ing cells, the nucleus is well-marked, but the chlorophyll-granules, which are very defi- 

 nite in the ordinary epidermal cells, are almost wanting, or at least, very poorly defined. 



