ORGANS OF NUTRITION. 



67 



but remain distinct and of a limited size. Sucli vascular bundles 

 are therefore named definite or closed vascular bundles. 



Fig. 15 



Fig. 159. 



\-\ 



Fig. 158. Transverse section of a flbro-vascular bundle of an Exogenous 

 stem (Melon), p. Pith. sv. Sinral vessels, vir. Medullary rays. w. 

 Wood. d. Pitted vessels, c. Cambium layer. I. Liber. Ic. Latici- 

 ferous vessels, ce. Cellular portion of the bark. e. Epidermal tissue. 

 . Fig. 159. Transverse section of a flbro-vascular bundle of an Endo- 

 genous stem (Palm), the upper portion being directed to the centre. 

 w. Wood-cells, sv. Spiral vessels, c. Cambium layer. d. Pitted 

 vessels, p. Parenchyma. I. Liber-cells. Ic. Laticiferous vessels. 



In Acrogenous stems the vascular bundles are chiefly made up 

 of vessels of the scalariform, annular, or spiral type, according 

 to the different orders of Acrogenous plants from whence they 

 have been derived; these are surrounded by delicate tubular 

 cells, and the whole is enclosed by a firm layer of wood-cells. 

 Such bundles only grow by additions to their summits, and as 

 the elements of which they are composed are not formed in suc- 

 cession like those of indefinite and definite vascular bundles, but 

 simultaneously, they are called simultaneous vascular bundles. 



The distinctive appearances which we have thus seen to occur in 

 the stems of the three plants above noticed are also accompanied 

 by certain differences in the structure of their embryo. Thus 

 [ plants with Exogenous stems have an embryo with two cotyle- 

 dons (figs. 14 and 15, cc); those with Endogenous stems have 

 1 but one cotyledon in their embryo (fig. 16, c); while those with 

 I Acrogenous stems have no proper embryo, and consequently have 

 \ no cotyledons. Hence exogenous stems are also termed Dicotyle- 

 i f2 



