570 Comparative Physiology. 



structure. It is very interesting to remark that the ducts of 

 young plants often present the appearance of those of ferns, 

 having the spiral fibre disposed within them, and afterwards 

 are converted into dotted ducts; showing that these are often 

 formed upon the type of a spiral vessel, and reconciling the 

 conflicting accounts of some who maintain they are of vascular, 

 and others that they are of cellular, structure." 



DeCandoUe, in his Vegetable Physiology, says some con- 

 sider life or excitability to be diffused through the organs, cells, 

 and vessels ; many that it exists in the vessels only ; but he 

 himself is of opinion that it exists principally, if not wholly, in 

 the cells. Life can be carried on without vessels, while it is 

 never done without cells. It is now found, he says, that the 

 vessels, once supposed contractile and to carry sap, convey air 

 only ; and that in most cases the sap ascends by the intercel- 

 lulary ducts ; the supposed peristaltic motion of the vessels of 

 Saussure being now asci'ibed to the cellular tissue ; it is, 

 therefore, the contractile force of the cells which causes the 

 motion of the sap. The contractile force in cells is, he says, a 

 modification of the systole and diastole of the heart, causing the 

 emission of milky fluids when the cells are irritated, and the emis- 

 sion of the same from both ends of a piece of the stem, Hum- 

 boldt having found it did not flow from pieces of the stem of 

 euphorbia killed by electricity. The vital vessels of Schultz, 

 which contain milky juices, are, he says, cells, and possess great 

 contractility. Also, when plants are killed by poisons, the cells 

 are found destroyed, while the vessels are often unhurt. The 

 dotted ducts he denominates elongated cellules ; but, as neither 

 these nor the tubes have valves, as in animal veins, they are 

 not fitted for the ascent of the sap ; and as the sap would cir- 

 culate through the cells with difficulty, it must take place, he 

 says, through the intercellulary passages, principally along the 

 woody fibre. The cells contract to allow the sap to ascend, and 

 again expand and act as valves in maintaining what has as- 

 cended. M. Bischoff, he says, in experimenting with coloured 

 solutions, found the colour to infuse into the vessels when the 

 water was boiled, but, when the water contained air, the vessels 

 were filled with the air and did not admit the colour. Mr. 

 Knight was once of opinion that the ascent took place through 

 the alburnous tubes : but, having found them full of air alone 

 when the sap was rising briskly, he experimented by stopping 

 the alburnous tubes and vessels ; and, still finding the coloured 

 infusions to rise, he concluded that the sap moved through the 

 cells of the cellular matter, but that, when a great flow of sap 

 takes place, it may permeate the walls of the tubes and vessels, 

 which, though usually containing air, appear thus filled with 



