XVI.] THE BEAN PLANT. 465 



intercellular passages, which are widest in the leaves, but 

 extend thence throughout the whole plant. 



The blade of the leaf is traversed by the branched vascular 

 bundles, the xylem being turned towards the upper, and the 

 phloem towards the lower surface. The parenchyma of the 

 leaf, or mesophyll, is of two kinds; towards the upper surface 

 the cells are closely packed, and elongated at right angles 

 to the surface, forming the palisade parenchyma. Towards 

 the lower surface the cells are of more irregular shape and 

 very loosely arranged, and are termed the spongy paren- 

 chyma. Both kinds of cells contain chlorophyll-grains, but 

 they are most abundant in the palisade cells. 



The root has an epidermis, bearing unicellular root-hairs. 

 Within this is a wide cortex of parenchyma, while the centre 

 of the root is traversed by a single vascular bundle, of 

 radial structure, usually containing four groups of xylem, 

 and four of phloem, which alternate one with another. The 

 lateral roots arise endogenously , immediately outside the vas- 

 cular bundle, opposite the xylem groups. They thus have 

 to force their way through the whole of the cortex before 

 reaching the surface. There are in typical cases four rows 

 of lateral roots, corresponding to the four xylem-groups 

 opposite which they originate. 



The difference between a flowering plant, such as the 

 Bean, and a flowerless plant, such as the Fern, at first sight 

 appears very striking, but it has been proved that the two 

 are but the extreme terms of one series of modifications. 

 The anther^ for example, is strictly comparable to a leaf 

 bearing sporangia, the sacs in which the pollen is contained 

 answering to the sporangia themselves. The pollen grains 

 exactly resemble spores in their mode of development and 

 answer to the small spores of those flowerless plants in which 

 the spores are of two kinds — some spores giving rise to 



M. 30 



