DEVELOPMENT OF THE OSTRICH FERN. 33 



the younger pai-ts of the petiole these cells are often much broader than long, owing- to 

 the rapid division. They contain fine grannlar protoplasm and distinct chlorophyll gran- 

 ules which are most abundant in the center of the cell, surrounding the large round nu- 

 cleus. The subsequent thickening of the petiole is brought about by the enlargement 

 of these cells which ultimately also increase very much in length. As the cells enlarge, 

 they lose the chlorophyll which seems to be gradually replaced by starch. Even when 

 colored with chlorophyll, the action of iodine caused the chlorophyll bodies to turn a 

 dark bluish color, while in the older cells, from which the chlorophyll has entirely dis- 

 appeared, the characteristic blue color is produced in the granules, which seem now to 

 have the whole of the chlorophyll replaced by starch, appearing, indeed, like ordinary 

 starch grains. A similar, apparent ti-ansformation of the chlorophyll bodies into starch 

 grains was observed in the jjrothallium. 



The fibro-vascular bundles of the petiole are ribbon-like, the transverse section being 

 several times longer than broad. Before any markings are evident on the walls of the 

 tracheids, their position can be easily made out by the greater size and transjjarency of 

 the procambium cells from which they are to be formed. These cells form a band run- 

 ning through the center of the bundle, and curving at the ends (seen in transverse sec- 

 tion) . The first tracheids are formed at the points of curvature on the inner side of the 

 central band of cells. From these the formation continues, following the course of the 

 baud.. These first ones are short and marked reticulately with close coiled spirals. The 

 secondary tracheids are mainly scalariform. As already noticed the cross-section of the 

 bundle, is much longer than broad, the proportion being about ten to one. It is very 

 slightly curved and o]ily about half as broad in the middle as at the ends; the outer 

 end being the broader, and being also less obtuse. The bundle is surrounded bj^ a bun- 

 dle-sheath comjoosed of small cells Avhose walls are colored brown. This is more plainly 

 seen in the upper pait of the petiole where the bundles are somewhat smaller. AVlien 

 the petiole becomes dry the cells of the bundle-sheath split through the middle, so that 

 the bundle separates from the ground tissue. Lying inside of the bundle-sheath, and 

 surrounding the tracheary tissue, are from six to eight rows of cells in the lower part of 

 the jietiole, a somcAvhat smaller number in the bundle of the more slender upper portion. 

 These vary much in size, are of prismatic form Avith moderately thick colorless walls, and 

 no intercellular spaces. In the loAver part of the biuidle these are filled Avith starch but 

 in the upper parts are nearly destitute of gramdar contents. The central part of the 

 bundle is occupied by the tracheary tissue, mainly composed of the large secondary 

 scalariform ti'acheids. They are strongly angiilar, the Avails betAveen neighboring ones 

 being straight. The division walls are very sharply defined and in the older bundles 

 often assume a clear yellowish broAAii color. In the narroAver part of the bundle they 

 form a central band of varying Avidth, the tracheids also differing a good deal in diame- 

 ter: but in the enlarged ends of the section the band of tracheary tissue, instead of ter- 

 minating at the end, is bent on itself forming often a double band. This is more strongly 

 marked in the outer end than the inner. The space betAveen the tAvo bands of tracheids 

 at the ends of the bundle is filled Avith cells nuich like those that lie just outside them, 

 but in the larger end the}' are soincAvhat smallei". In some jjlaces small cells lie between 

 tlie tracheids, which in tlie ])arts of the petiole that survives the winter become filled Avitli 



JIICMOIKS UOSTON SOC. NAT. HIST., ^ t)L. 1\. 



