differences in the Structure of Stems. 241 



developed until at last they almost form a continuous mass, 

 the parenchyma which previously separated them is thereby 

 compressed into some small insulated patches, that appear 

 scattered through the completely formed wood in little narrow 

 vertical bands, which, in regard to their origin, may rightly 

 be termed vertical medullary rays. On the outside of these 

 cords are found in the wood very frequently spiroidce still un- 

 altered, forming the commencements of the outer vascular 

 bundles. I have pursued the entire development of this pecu- 

 liar structure in two species ofPisonia, in Amaranthus viridis, 

 Beta Cicla, Atriplex hortensis, Chenopodimn Quinoa, ^c. Many 

 other plants of the families mentioned, such as the Piperacecs, 

 which I could only examine under certain circumstances, 

 prove, by their structure, that this peculiarity is quite general 

 in those families. 



A curious form of wood probably belongs here, (and per- 

 haps the whole family of the Crassulacece), but I had no op- 

 portunity of following up the history of their development*. 

 In the old stem of an undetermined Echeveria I found, for 

 instance, an entire uniform mass of wood, formed of prosen- 

 chymatous cells without vessels, and scattered therein small 

 vertical cords of a very delicate-walled parenchyma, in the 

 midst of which ran spiral vessels, most of which might still be 

 unrolled. 



3. A third point of importance, arising from the essential 

 differences of stems, is the relation of the axis to the parts 

 given off from its periphery, the leaves and buds. Hereto be- 

 long a multiplicity of phenomena. 



A. A phaenomenon common to all Dicotyledons, is the 

 formation of nodes. A lateral organ in fact originates uni- 

 versally among Dicotyledons only from the nodes ; the part so 

 called in botanical descriptions is not here intended (for that 

 is mostly a crude conception of a somewhat isolated form of 



* I beg here expressly to observe that the earlier stages were not at my 

 disposal, and protest solemnly against the reproach of having overlooked 

 {iuything, if the development should give any other result. 1 should not no- 

 tice this if Meyen (in his Annual Report for 1838, p. 44) had not so ground- 

 lessly charged me with error, though I had explicitly declared that I had 

 been unable to examine the earlier stages, and where, moreover, the history 

 of the development shows that my supposition on the signification of the 

 questionable formation was correct. 



