PHENOMENA OF PLANT-LIFE. 25 



how these spongioles of the leaf send down the elaborated sap, 

 one channel sufficing for the transit of both." This hypothesis 

 concerning the circulation of sap is accepted only by its inventor, 

 and is directly opposed to most of the facts of plant-growth. 



Finally, Dr. Pettigrew has conceived a system of syphons 

 by the aid of which he is able to account to his entire satis- 

 faction for all he knows concerning the circulation of sap. 

 He says : "The vessels which convey the sap, as is well known, 

 are arranged in more or less parallel vertical lines. If the 

 vessels are united to each other by a capillary plexus, or, what 

 is equivalent thereto, in the leaves and roots, they are at once, 

 as has been shown, converted into syphon-tubes, one set 

 bendinir upon itself in the leaves, the other set bending upon 

 itself in the roots. As, however, a certain portion of the 

 syphon-tubes, which bend upon themselves in the roots, 

 are porous and virtually open towards the leaves ; while a 

 certain portion of the syphon-tubes, which bend upon them- 

 selves in the leaves, are porous and virtually open toward 

 the roots, it follows that the contents of the syphon-tubes 

 may be made to move by an increase or decrease of moisture, 

 heat, etc., either from above or from below. In spring, the 

 vessels may be said to consist of one set, because at this period 

 the leaves and the connecting plexuses which they contain 

 do not exist. All the vessels at this period may, therefore, 

 be regarded as carrying sap in an upward direction to form 

 shoots, buds and leaves, part of the sap escaping laterally, 

 because of the porosity of the vessels. In summer, when the 

 leaves are fully formed, the connecting links are supplied by 

 the capillary vascular expansions formed in them, the tubes 

 are in fact converted into syphons. As both extremities of 

 the syphons are full of sap in spring and early summer, an 

 upward and a downward current is immediately established. 

 When the downward current has nourished the plant and 

 stored up its starched granules for the ensuing spring, the 

 leaves fall, the syphon structure and action is interrupted, and 

 all the tubes (they are a second time single tubes) convey mois- 

 ture from above downward, as happens in autumn. As the 

 vascular expansions or networks are found also in the stems 

 of plants, it may be taken for granted that certain of the tubes 

 are united in spring, the upward rush of sap being followed 



