ANATOMY or PLANTS. 1S5 



cotyledons wliich embrace the young plant during its growth, 

 and its consequence is the parallel progress of the nerves in 

 the leaves, without veins, and without a reticular distribution. 

 In the Ferns alone we observe a construction difterent in 

 this respect, that strong bundles of numerous vasa scalaria 

 are intermingled with the sap-vessels ; and, being surrounded 

 by a peculiar brown cellular membrane, they stand in definite 

 number and in fixed order between the rind and the pith. 

 Here, also, the nerves of the leaves soon pass into veins, and 

 into numerous ramifications. 



291. 



In all perfect plants there is formed, where the two seed- 

 lobes embrace the rising plumula, and out of the knot which 

 vmites these, a connected circle of spiral-vessels and sap-vessels, 

 which rises perpendicularly between the pith and the bark, 

 and thus forms the concentrical layers of the parts of the stem. 

 In the knots of the stem this circle is inteiTupted, at the same 

 time that the cellular texture which is there crowded, affords an 

 opportunity for the production of new spiral-vessels and sap- 

 vessels. In the mean time a similar circle proceeds from the 

 knots upwards. The first spiral-vessels which take their 

 place are always the innermost: these maintain, for a long 

 time, their original shape, and even their green colour. 

 Those of later growth take their place more towards the 

 outer parts of the stalk, have a greater disposition to become 

 woody, and shew this by their speedy transition into vasa 

 scalaria and punctured vessels. 



292. 

 It follows from this, that the innermost and outermost layer 

 of the stem is purely cellular, while the middle ring, on the 

 contrary, is composed of the higher primitive forms. The 

 outermost layer is called the Rind ; it consists of the proper 

 rind and the epidermis, which covers the former. The latter 

 is probably formed by the deposition of sap, and by its hard- 

 ening under the influence of the ingredients of tlic air. It 

 has always a different colour from the proper rind : it is, for 



