70 



SPIRAL DUCTS. BRANCHING SAP-VESSELS. 



originated, of a simply membranous character ; but more com- 

 monly, their walls are fortified by an interior deposite, which 

 does not, however, entirely line them, but leaves the membrane 

 bare at certain points, giving that dotted appearance already 

 described in treating of the cells (. 74). Hence these vessels 

 are commonly termed dotted ducts. It is through such as these, 

 that the sap principally rises in the stem and branches, and is 

 conveyed to the leaves. They are by far the largest vessels 

 contained in the vegetable fabric ; and their open mouths are 

 visible in almost any stem, when cut across. They are of par- 

 ticularly great diameter, when the stem itself is small and long, 

 but bears a considerable amount of leaves, as is the case in the 

 Vine and the common Cane ; in these, their orifices at once strike 

 the eye ; and, if the stem of a growing plant be cut across, the 

 oozing of the sap from their mouths will be easily distinguished. 

 86. The office of all these ducts is the same, that of con- 

 veying fluid. It is only in the true spiral vessel, that we find 

 air. These varieties have been described with somewhat greater 

 minuteness than may appear necessary; because the young ob- 

 server who examines the vegetable structure for himself, as it is 

 hoped that many will be led by these pages to do, will be 



liable to be perplexed by meeting 

 with them, if not previously ac- 

 quainted with their characters. 



87. One other form of ele- 

 mentary tissue now remains ; and 

 this differs from all the rest. It is 

 a system of branching vessels, con- 

 fined to the under side of leaves, 

 and to the bark, and serving only 

 for the conveyance of the nutritious 

 sap ; which is carried by it from 

 the leaves, where it is produced, 

 down the bark, and thence to all 

 parts of the structure. The walls 

 ^r*S: f these' branching vessels are ex- 

 LATEX OR NUTRITIOUS juics. trcmely delicate, so that they can 



