TISSUES AND SIMPLE ORGANS. 



121 



among dicotyledons. The leaf-sheaths of Graminece, Cyperacece, 

 and of EquisetcB perform this function by enclosing the growing 

 (as a rule the basal) portion of the internode from which the node 

 projects after it has become sufficiently strengthened. The nodes 

 of grass-stems, and also the region above, are interesting structures 

 for investigation and study; some of their peculiarities, especially 

 geotropic curvatures, will be discussed later. Fig. 67 and 68 will 

 aid in explaining what has just been said and that which is to 

 follow. 



In Fig. 68 the typical mechanical tissue which has lost the 

 power of growth is colored black. The supporting tube (leaf- 

 sheath) s encloses the yet weak and 

 cambial base of the stem h ; s extends 

 above the cambial portion of the inter- 

 node. At co the mechanical tissue- 

 system is not composed of typical bast 

 but of collenchyma, therefore capable 

 of growth. If a grass-stem is placed 

 in a horizontal position, the more rapid 

 growth of the lower side of this collen- 

 chymatous node will cause it to rise to 

 a vertical position. The firm and more 

 mature portion of the stem within co is 

 thereby passively bent. 



A rare case, occurring so far as 

 known only in the genus Armeria, 1 

 has been observed and may be briefly 

 mentioned. The mechanical sheath of 

 bracts at the base of the inflorescence 

 extends from above downward ; the 

 growing part of the peduncle lies at the 

 upper end of the internode. The 

 sheath is completely formed in the 

 young plant, and after the peduncle has completed its growth it 

 dries up and finally becomes torn. 



FIG. 68. Longitudinal section 

 through the node of a grass- 

 stem. 

 (Diagramatic after Schwendener.) 



1 Reported by the author in 1881 (Monatsber. der Berl. Akad.). Professor 

 Schwendener, who has doue so much for scientific teleology, during on<? of his ex- 

 cursions in the vicinity of Berlin, expressed an opinion, as to what was probably 

 the true state of the case, which led me to make more exact investigations. 



