SECRETORY PASSAGES 525 



Cotinus, B. suaveolens, II. glauca, etc.), the leptome-strands contain 

 secretory passages; in Pinus Laricio and certain other Conifers, on the 

 other hand, these structures are found in the primary hadrome-strands 

 of the stem. In the majority of cases, however, the secretory ducts are 

 not actually located in the vascular bundles, but are merely associated 

 with them in a characteristic and constant manner. In Umbelliferous 

 steins, for example, each vascular bundle (or, at any rate, each of the 

 larger strands) is accompanied by a cortical oil-passage. In Achillea 

 millefolium, Cirsium arvense, Tanacetum vulgare and other Compositae, 

 a duct runs alongside of each leptome-strand ; in Solid ago limonifolia 

 each leaf-trace bundle is provided with two oil-canals, one opposite the 

 hadrome-, and the other opposite the leptome-strand ; in Helianthus 

 annuus, finally, every bundle is subtended by a crescentic group of 

 oil-passages, both on its inner and on its outer side. 



In conclusion, it will be necessary to consider the physiological and 

 ecological significance of secretory passages. In view of the fact that 

 these structures are so frequently associated with vascular bundles, 

 and in particular with leptome-strands, it might at first sight seem 

 probable that they serve for the reception of useless waste products, 

 which travel towards the leptome-strands and other vascular tissues 

 from the various tissues that are engaged in active metabolism; but 

 the chemical character of the contents of secretory ducts hardly 

 accords with this notion. 



The reason for the frequent association of secretory passages with 

 leptome-strands and other vascular tissues is therefore, in all proba- 

 bility, an ecological one, as has, in fact, been suggested by Stahl and 

 Kniep. The substances contained in these passages are often of such 

 a kind as to be capable of affording " chemical protection " against 

 noxious animals ; hence small assailants which have penetrated into 

 the interior of an organ will be more or less effectually discouraged 

 from attacking the conducting strands, the continuity of which is so 

 vital to the well-being of the plant if the latter are protected by a 

 series of secretory ducts (or excretory sacs). 



Among woody plants, the resin- and gum-passages which occur in 

 the cortical parenchyma (and sometimes also in the woody cylinder) of 

 the stem, probably often serve to cover up wounds with an airtight 

 layer of secretion ; in this way the stem is rendered less liable to 

 infection, and consequent decomposition, by fungoid parasites. This 

 interpretation is strongly supported by the results that follow, when 

 incisions are made in Coniferous stems for the purpose of collecting 

 resin on a commercial scale. Here enormous surfaces become coated 

 with the secretion, while the subsequent healing proceeds very satis- 

 factorily. According to J. Moeller, balsam-ducts may actually be 



