PROTECTION OF INTERCALARY MERISTEMS 183 



growth. 101 



enlargement 



marked 



woody plants, on the other hand, it is generally cast off owing to the 

 formation of cork, when the twig has reached a sufficient thickness. 



Organs which are engaged in intercalary growth may be strengthened 

 in a totally different manner, namely by local enlargement of the 

 internode in the region of most active growth. In Tradescantia 

 erecta, where an intercalary growing-zone is situated at the base of 

 each internode, the diameter of the stem is nearly twice as great at 

 these points as it is at the upper ends of the internodes. The converse 

 relation is illustrated by the peduncles of certain Compositae, in 

 which intercalary growth is most active towards the distal end of the 

 organ. It was Westermaier who pointed out that the unusual shape 

 of these peduncles is correlated with 

 their peculiar type of intercalary 

 In such cases the local 

 is sometimes very 

 The peduncle of Arnoseris 

 minima, for example, may be six 

 times as thick immediately below 

 the capitulum as it is near its base. 

 A zone of intercalary growth may, 

 finally, be located near the middle 

 of an internode. This somewhat 

 unusual condition is exemplified, 

 according to Westermaier, by Pilea 

 oreophila : here, accordingly, it is 

 the middle of the internode that is 

 enlarged. 



The most perfect arrangements 

 for the mechanical protection of 

 intercalary growing-zones are found 

 in the haulms of Giiamineae 



and Cyperaceae. In these organs there is, at the base of each 

 internode, a relatively short zone which remains permanently meriste- 

 matic, thus constituting an intercalary growing-point. The zone 

 in question is of course soft and mechanically weak. In order, 

 therefore, that the base of the internode may not be broken by the 

 first puff of wind, or by the mere weight of the overlying portion 

 of the haulm, it must be strengthened by means of some special 

 mechanical device. The requisite protection is provided by the 

 sheathing leaf-base, which completely encloses the whole meristematic 

 and growing region of the internode. The mechanical system of 

 the leaf -sheath takes the place of the still undeveloped skeletal system 

 of the ensheathed portion of the stem. Since the leaf-sheath is tubular, 



Fio. 64. 



T.S. through the leaf-sheath of Brachypodium 

 sylvaticum, "5 cm. above the node. Subepidermal 

 fibrous strands with apposed mestome-bundles. 



