THE SEEDLING AND YOUNG PLANT. 81 



formed in the cells of the leaves. The water, with salts 

 in solution, coming from the soil after it has been ab- 

 sorbed by the root-hairs, passes up the wood (xylem) of 

 the roots and stem, through the vessels of the petioles 

 and leaf -venation, and is finally distributed to the cells 

 of the mesophyll ; the substances formed in these cells 

 then pass down by the phloem (sieve-tubes, etc.) of the 

 venation and leaf -stalk, and thence are distributed to 

 other parts of the plant. 



Now let us look at the mesophyll which these vascu- 

 lar bundles support and serve as conduits for. It con- 

 sists of two distinct parts (Fig. 22). Beneath the upper 

 epidermis, the cells of which are fitted closely together 

 without intercellular spaces and are devoid of chlorophyll 

 corpuscles, there are one or two rows of vertical sausage- 

 shaped cells, closely arranged like the wooden railings 

 of a complete palisade consequently they are termed 

 the palisade cells. The lower moiety of the mesophyll, 

 on the other hand, is composed of irregular cells with 

 large intercellular spaces between them, and this loose, 

 spongy tissue, as it is aptly called, abuts below on the 

 lower epidermis. Both the palisade cells and those of 

 the spongy tissue contain numerous chlorophyll cor- 

 puscles, as said. 



This lower epidermis is worth a few minutes' con- 

 sideration. It, like the upper epidermis, is also com- 

 posed chiefly of closely fitting cells devoid of chlorophyll 

 corpuscles, excepting that here and there we notice pairs 

 of smaller cells containing chlorophyll each pair with 



