ANATOMY OF PLANTS. 



177 



In more perfect plants they are found, for the most part, 

 in the neighbourhood of the spiral vessels : they constitute 

 the basis of trees, and a great part of the young wood, 

 and shew a toughness and a power of resisting violence, 

 which, considering their fineness, is astonishing. That they 

 arise from the first form, cannot be believed, because they 

 proceed directly from the generative sap, hke fine straight 

 tubes, close to the spherulae. But the stretched form of the 

 cells is very like the tube-form. It is even undeniable that 

 it constitutes, especially in the lower organic bodies, the 

 transition-form from the cells to the tubes. In the fruit-stalk 

 of the Musci Hepatici and Frondosi we have not yet disco- 

 vered the proper tube-form, but only stretched cells, similar 

 to tubes, which apparently answer the purpose of these latter 

 bodies. 



276. 



The object of Nature, in the formation of tubes, seems 

 simply to be, by means of them to lead upwards the unpre- 

 pared sap. The similarity of the sap-tubes to hair-tubes 

 leads us to consider them as a physical contrivance, by which 

 the ascent of the sap is assisted, although the only principle 

 upon which these last act cannot obtain a place in this struc- 

 ture, (376.) 



The pointed extremities of these sap-tubes present some 

 difficulty to this account. They lie with their ends oblique- 

 ly placed to one another, and the ascent would seem to be in- 

 terrupted by this position, if v/e did not here also admit the 

 organic perspiration of the sap through partitions which in 

 themselves are impervious. 



C. On the Spiral Vessels. 



m. 



This is called the spiral form, because originally it consists 

 of canals, the sides of which are entirely formed by spiral 



M 



