424 CONSERVATIVE ORGANS. [LECT. VIII. 



of air introduced into the centre of the stem 

 at a period subsequent to the formation of the 

 pith, which at first fills up the whole of the space 

 within the vascular circle. Whether the air con- 

 tained in these stems be introduced from without 

 or generated in the stem as a secretion, is still 

 a question in phytology. Examining it eudiome- 

 trically, I found that the stem of Cow Parsnep, 

 Heracleum sphondylium, contains a little more 

 carbonic acid gas, than atmospherical air, from 

 which it differs in no other respect. But if it be 

 a secretion, the components will probably vary in 

 different stems *. 



b. The internal structure of the partitioned 

 hollow stems is the same as that of the fistulous, 

 except at the knots where the partitions are si- 

 tuated. In selecting the stem of Common Hemlock, 

 Conium maculatum, to illustrate this part of their 

 structure, I shall point out to you a variation in 

 the arrangement of the fasciculi of descending ves- 

 sels, which is peculiar to this plant and a few others 

 of the same natural order of the umbelliferae. 



If we place a transverse section of the stem 

 of Hemlock under the microscope, we find the 

 cortex to consist of a cuticle composed of a thin 

 cribriform epidermis,, covering two rows of square 

 cells 0. (Plate 9, fig. 8) ; and a cellular integu- 

 ment, c., through which the proper vessels descend 



* This part of our subject shall be fufly discussed in its pro- 

 per place. 



