ANGIOSPERMS. DICOTYLEDONS. 465 



again with the succeeding period of vegetation, as in our native woody plants, a 

 layer of wood is formed in each period of vegetation accompanied usually by a 

 layer of secondary phloem ; this woody layer is distinctly marked off from that of the 

 previous and from that of the following year and is called the annual ring. The rings 

 are generally clearly distinguishable by the naked eye, because the wood formed at 

 the beginning of each period of vegetation, being of looser consistence and having a 

 larger number of vessels especially in angiospermous trees, has a different appearance 

 to the more compact wood of autumn. The cells of the spring wood are broader 

 than those of the wood formed in autumn, and their radial diameter especially is 

 greater ; the latter appear to be compressed from within outwards and broad in 

 the tangential direction ; their lumen is smaller, and the area of wall is therefore 

 greater on a similar transverse section, and consequently a given quantity of autumn 

 wood is denser than the same quantity of wood formed in the spring 1 . While 

 Dicotyledons differ widely from Monocotyledons they agree almost exactly with 

 Gymnosperms in this mode of growth in thickness, only the latter have no shortly 

 articulated vessels with small pits in the secondary wood ; in this point however 

 Ephedra affords a transition to the Dicotyledons (Mohl) ; Dicotyledons also show 

 a certain advance in organisation in the greater variety of forms in the cells of 

 which the xylem and phloem are composed. 



Very striking deviations from this normal structure are to be found in the Sapin- 

 daceae ; some species of the order are formed in the usual manner, but in others 

 the transverse section of the stem shows in addition to the usual ring of wood a 

 number of smaller closed rings of different sizes in the secondary phloem, each of 

 which increases in thickness like the ordinary ring by means of a layer of cambium. 

 Nageli supposes the principal cause of this to be that the primary vascular bundles 

 of the stem do not lie in a circle on the transverse section, but in groups more 

 towards the outside or inside. When the interfascicular cambium is formed in the 

 fundamental tissue, the isolated bundles are connected together according to their 

 grouping on the transverse section into one closed ring in Paullinia, or into several 

 in Serjana. 



Many and various deviations from the normal structure of the stem 2 in different 

 families are caused by the primary bundles not being arranged in a single ring ; they 

 may even appear to be distributed without any order on the transverse section. ' These 

 exceptions to the usual structure occur either in quite isolated species in genera and 

 families in which the structure is normal, as in the Umbelliferae, or in a number 

 of species of genera of typical construction, as Begonia, or they are characteristic 

 of certain genera or smaller families, as Nymphaeaceae, Calycanthaceae, Podophyllum, 

 Diphylleja, more rarely of large families, as the Piperaceae and Melastomaceae ; 

 in the latter there are exceptions to the grouping of the bundles which prevails 

 in the majority of the allied forms.' These exceptions are due to the radial oblique 

 direction of the leaf-trace-bundles or to the appearance of cauline bundles in addition 

 to the common bundles disposed in the ring : 



a. Medullary bundles. 



i. All the bundles are leaf-trace-bundles, some arranged after they have entered 

 the stem in the typical ring and having a radially perpendicular course in it, 

 others penetrating further into the stem and therefore appearing in the pith, and 



1 The cause of this difference, as Sachs formerly suggested in the first edition of the Lehvbnch 

 and De Vries has ascertained by experiment, is the variation in the pressure exerted by the cortex on 

 the cambium and the wood. This pressure is less in spring and is constantly increasing in autumn. 

 See De Vries, Flora, 1872, No. 1 6. 



2 De Bary, Vergl. Anat. p. 258 ; the remarks are taken, partly literally, from De Bary's 

 account. 



[2] H h 



