OBSERVATIONS UPON THE PHENOMENA OF PLANT-LIFE. 315 



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 up- 

 ward direction to form shoots, buds, and leaves, part of the sap escaping lat- 

 erally, because of the porosity of the vessels. In summer, when the leaves are 

 fully formed, the connecting links are supplied by the capillary vascular ex- 

 pansions 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 downward current is immediately established. When the down- 

 ward current has nourished the plant and stored up its starched granules for 

 the ensuing spring, the leaves fall, the syphon structure and action is inter- 

 rupted, and all the tubes (they are a second time single tubes) convey moisture 

 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 by a slight downward current, as happens in endosmose and exosmose. 

 As, moreover, the spongioles of the roots and the leaves are analogous struc- 

 tures, and certain tubes are united in the roots, the downward current in 

 autumn is accompanied by a slight upward current. This accounts for the 

 fact that at all periods of the year, the upward, downward and transverse cur- 

 rents exist ; the upward and downw^ard currents being most vigorous in spring 

 and autumn, and scarcely perceptible in winter. Furthermore, as some of the 

 vascular expansions in the leaves are free to absorb moisture, etc., in the same 

 way that the spongioles are, it follows that the general circulation may receive 

 an impulse from the leaves or from the roots, or both together, the circulation 

 going on in a continuous current in certain vessels." 



This original effort of the learned lecturer on physiology, at Surgeons' Hall, 

 in Edinburgh, published in 1874, to explain some of the most difficut problems 

 of vegetable life by a mere hypothesis, which assumes that sap flows in the 

 vessels ; that there are spongioles in the leaves which absorb water ; that the 

 sap descends to the roots and escapes from them in autumn; and that an 

 imaginary system of syphons does all these wonderful things, which have not 

 been proved to occur at all, and which well-informed physiologists are almost 

 unanimous in denying, reminds us of the adage that " a prophet is not without 

 honor save in his own country." This is not the method of the Baconian 

 philosophy. 



CIRCULATION OF SAP. 



In the observations which follow, we hope to add some new facts to the 

 knowledge of the world concerning the phenomena of plant-growth ; but are 

 painfully conscious of the need of much more investigation before a complete 

 and correct theory of the circulation of sap can be stated. Exceptions have 

 been taken to the use of the expression " circulation of sap ; " but since there 

 is an evident distinction between the crude and elaborated saps, both in their 

 composition and their location in the plant, at least in the higher forms of 

 vegetation, and since the circulation of blood is accepted as a proper term even 

 Avhen applied to animals without a heart, we prefer to retain it in our vocab- 

 ulary. 



OSMOSE. 



In regard to the causes Avhich induce the absorption of water and soluble 

 substances by the roots of living plants, it seems unfortunate that so much has 

 been claimed for osmose in this connection. Boussingault has recently shown 



