Some Characters of Xylem Tissue in Cycads
The detailed investigation of certain anatomical features of the Cvcads has been undertaken in the hope of throwing light on the origin of the more specialized structures occurring in the higher Gymnosperms. In recent years considerable work has been devoted to determining the details of anatomical structure in the Conifers. These details have been given much prominence as evidences of the inter-relationships of the various groups. A lack of knowledge of the ancestry of the structures
... tructures themselves, however, has minimized their value as criteria in phylogenetic investigations. This knowledge can be supplied only by a study of primitive forms. Pitting The shape and arrangement of bordered pits in the woody tissue have long been regarded as valuable phylogenetic data. It is largely owing to these features that the Araucarians have been supposed to be closely related to Cordaitean forms, and many botanists still hold this view, notwithstanding the arguments advanced by JEFFREY (6) and SEWARD (9) in favor of different lines of ancestry for the family. In I907 GOTHAN (4) worked out a phylogenetic line of development of bordered pitting, considering the most primitive type to be hexagonal and crowded over the whole tracheid wall. According to his theory, the pitting next became eliminated from the tangential walls, but still covered the radial as before. A further elimination resulted first in small isolated groups of pits, then in the uniseriate flattened condition, and finally in the scattered arrangement, where the pits occur singly on the tracheid wall. This series of eliminations took place on the middle part of the wall of the tracheid, the crowded arrangement being retained on the ends to facilitate vertical movement of the sap. GOTHAN found his types of arrangement combined in a fossil plant, but in no living form. JEFFREY is more conservative than many others in his estimate of the significance of pitting, but considers that it is of distinct value in classification when its character in all parts of the plant is considered. On this ground, in his work on the Araucarineae, he accepts the presence of opposition pitting in the cone axis, and scattered pits in the seedlings, as denoting descent from an Abietineous type. He has neglected the character of the pitting in primitive forms such as the Cycads, however, and his interpretation is not in harmony with the facts which these forms disclose. This was done, notwithstanding the fact that as early as i840 DON (2) recognized the value in phylogeny of the study of Cycads. He carefully worked over the character of tracheids by such methods as were in vogue at that time, and agreed with MEYENS, a still earlier investigator, that the spiral, scalariform, reticulate, and border-pitted types could be referred to a common origin. The importance of these transitions was emphasized also by PEN-HALLOW (7) in I907 as affording valuable data on the ancestral character of the bordered pit of the higher forms. In i919 BAILEY (i) argued that opposite pitting is formed by the breaking up of bordered scalariforms, and that the alternate type was formed from this by a "staggering" of the rows of pits. In this paper certain features of the primary wood of the Cycads will be considered first. Fig. I is a longitudinal section of the petiole of Cycas revolula, showing the tangential walls of the tracheids in the neighborhood of the protoxylem. The tracheid a shows the characteristic spiral and scalariform structure of the protoxylem. In transverse section (not figured) the scalariform bars are seen to arch over the intervening spaces so as to form very narrow borders. On the cell b the scalariforms are more closely approximated, and through the slits may be seen shorter pores, belonging to the adjacent wall of the next tracheid. The tracheid c also shows this clearly. In the other two tracheids typical bordered pits are present. The type of scalariform from which such bordered pits are formed is shown in fig. 2 . It is a scalariform similar to that formerly described, except that the borders are wider. Fig. 3 , another section from a Cycas petiole, indicates transitions in the formation of ordinary bordered pits from this This content downloaded from 080.082.