1. The average length of the sclerenchymatous elements in different leaves or leaf-sheaths of the plants examined is not constant, but depends in Sanseviera and Agave upon the length of the leaf in which the elements occur, and seems to depend in Musa upon the length of the pseudostem. The longer leaves (or leaf-sheaths) contain longer fibers, the shorter leaves (or sheaths) shorter sclerenchymatous elements. The shortest elements are found in the anastomoses running across in the leaf and connecting the longitudinal strands in Sanseviera and Agave. 2. The sclerenchyma fibers originate by repeated divisions of parenchyma cells in the growing zone of the leaf (or leaf sheath) as bundles of very thin walled, uninucleate elements, these elements having the same length as the initial parenchyma cells, but showing a smaller diameter. 3. In order to reach its definite length such a young fiber elongates until it has reached many times its initial length; as a maximum of stretching an elongation up to 70 times was observed (in the longest fibers of Agave and Sanseviera). As a rule the elongation amounts to 40 to 60 times the initial length in the longer leaves of Sanseviera and Agave and in the pseudostem of Musa. In the shortest sclerenchyma elements the mature length is sometimes only twice the initial length (in the anastomoses of Sanseviera and Agave). 4. During this process of elongation of the initial sclerenchyma fibers not the slightest gliding growth between — or along — other elements was observed. 5. The acute apices of the longer fibers are formed during the elongation of the cells from the transverse walls, which walls show in the beginning a slightly slanting position. The acuteness is the result of the difference in longitudinal and transverse growth-rate of the fiber. The longest fibers always show the most acute apices. 6. The secondary thickening of the cell wall occurs only when the fiber has reached its final length. The thickening occurs in every part of the cell wall simultaneously and equally. The thickening process soon ceases, after which the lignification begins. 7. In the primary cell wall of the sclerenchyma elements the direction of the long axis of the refractive-index ellipsoid is transverse in surface view of the cell. In the secondary cell wall of the longest fibers the direction of the long axis of this ellipsoid differs only little from the longitudinal axis of the fiber. In fibers of average length the long axis of the index-ellipsoid forms an angle of about 45° with the longitudinal axis of the fiber. In the shortest sclerenchyma elements the long axis of the index-ellipsoid shows a transverse direction. Between these three typical directions of the index-ellipsoid all transitions are present, if fibers of various lengths are examined. When the long axis of the refractiveindex-ellipsoid forms an angle of about 45° with the long axis of the fiber, the latter is apparently isotropic, this phenomenon being caused by the crossed systems in the upper and lower wall of the fiber. 8. The direction and shape of the pits in the secondary wall is also variable. In the longest fibers very steep inclined long pits are present, in fibers of average length the (long) pits form angles of about 45° with the long axis of the fiber and in the shortest sclerenchyma elements short, roundish-oval pits are found, which are placed in transverse direction. Between these forms and directions of the pits all transitions are also present. 9. The primary cell wall of the sclerenchyma fibers mostly shows in the initial stage, besides reactions on cellulose and pectic substances, the “amyloid”- and “collose”-reactions after Ziegenspeck. It seems highly probable that these reactions occur when the amount of cellulose is small (e.g. 10—12%), which may often be the case in young cell walls. 10. The secondary wall originally shows reactions on pectic substances and on cellulose. When the secondary wall is completely formed, lignification begins, occurring at first in the primary wall and middle lamellae and later on in the secondary thickening layers. 11. In the primary fiber wall as well as in the secondary one the pectic substances disappear during the process of lignification entirely or in part; the reaction on “lignin” with phloroglucinol-HCl becomes the more intensely red, the less the walls are stainable with ruthenium red. The process of lignification, as a rule, ceases as soon as the pectic substances have disappeared; sometimes the lignification ceases earlier, but never later. During the lignification in none of the three plants examined a considerable increase in thickness of the fiber wall was observed; this statement does not agree with the data of Alexandrov & Djaparidze (2), obtained from other objects. 12. The tracheids of the plants studied do not stretch by means of gliding growth along the sieve tubes, though they are, as a rule, longer than the elements of those sieve tubes; the differences in length are to be ascribed to the fact that the tracheids always begin to grow out earlier than the elements of the sieve tubes do. 13. In the unlignified secondary thickenings of the walls of the tracheids only cellulose and pectic substances could be indicated. During the lignification the reaction on pectic substances decreases as the amount of “lignin” becomes more considerable. The lignification ceases at the moment in which the pectic substances have disappeared completely. The lignifying cell wall does not increase in thickness; this fact does not agree with the data of Alexandrov & Djaparidze, obtained in other objects. 14. From the preceding it may be concluded that; a. in Monocotyledons which do not show secondary growth gliding growth does not occur, or, at least, occurs less generally than could be expected from the existing literature and b. during the process of lignification in the Monocotyledons studied probably no new cell wall substance (“lignin”) is laid down, but that this substance is formed on the very spot out of substances already present (the pectic substances). This investigation was started at the Botanical Laboratory of the Government University, Leiden, Director Prof. Dr. L. G. M. Baas Becking and continued at the Laboratory for Technical Botany, Delft. The author desires to express his indebtedness to Prof. Dr. G. van Iterson Jr. for hospitality and for his continuous interest in this subject.