26 HOFFMANN ON THE CIRCULATION 



the fluid, and on reaction formed sharply defined dark blue 

 lines, which could be easily traced even in the stem, both up- 

 wards and downwards, more than 1 foot from the absorbing. 

 branch. Microscopic investigation showed that the blue colour 

 had only spread half or the whole width of a row of cells, around 

 the tracheae. A longitudinal section of the (upper) axil of the 

 branch indicated the course of the trachese by curvilinearly ar- 

 ranged blue spots and streaks, and not by continuous blue lines. 

 Consequently these tracheae do not run so perfectly on one and 

 the same plane here, that their entire course can be at once dis- 

 played by a single section. It is worth notice, that only that 

 side of the stem on which the branch was seated was coloured 

 blue, the opposite not at all. 



That the pith does not conduct, is comprehensible, since the 

 air it contains could not readily be displaced by w^ater pene- 

 trating, and each of these cells containing air is a completely 

 closed sac. But it is difficult to understand why the medul- 

 lary sheath and the heart-wood do not conduct sap, since here 

 communicating unrollable spiral and striped tracheae are present 

 in abundance. This non-conducting layer amounted, at the 

 point of attachment of the branch, to a quarter of the woody 

 layer (the entire diameter of the stem at this point amounted to 

 1:J: inch). It is certain that the air visibly contained in these 

 tracheae is no hindrance, for it is none in those of the outer, 

 younger wood. The elongated cells of the medullary sheath 

 are far narrower and closer than the prosenchymatous cells of 

 the w^ood; they are crowded with starch- granules, w^hich are 

 absent from the rest of the prosenchyma of the wood up to the 

 medullary rays. Can it be some starchy or gummy mucilage 

 which stops the progress of the fluid ? I believe not ; there is no 

 dissolved starch present ; iodine distinctly reveals the unaltered 

 starch-granules. And as to gum, it is not evident how this 

 could close up the tracheae from the cells, since the microscope 

 shows the former to be filled wdth air-bubbles. 



6. Repetition of the preceding experiment. — On a sycamore 

 trunk 3 inches in diameter, a branch A, at a height of 5 feet, 

 was cut off, bent down, and fastened : the wound dipped in a 

 glass of 1 cubic inch capacity, filled with concentrated solution 

 of ferrocyanide of potassium. At the side (the south) of the 



