THE FOOD OF PLANTS. 11 



In order to gain general information as to the anatomical struc- 

 ture of the mesophyll, we now proceed to prepare as thin as pos- 

 sible transverse sections of leaves, and select for examination say 

 the foliage leaves of Dahlia variabilis, Vitis vinifera, Berberis vul- 

 garis, Syringa vulgaris, Trifolium pratense, Ilex, or Fagns sylvatica. 

 On microscopic examination of the sections, it is at once evident 

 that the green mesophyll is not of the same structure on the upper 

 and lower sides of the leaf. (See Fig. 3.) Below the epidermis of 

 the upper side of the leaf we find tubular cells, elongated at right 

 angles to the surface, and termed palisade cells, while on the 

 under side of the leaf is developed spongy parenchyma abounding 

 in intercellular spaces. The cells both of the palisade and of the 

 spongy parenchyma contain chlorophyll grains, but the former, 

 for reasons which cannot be set forth in detail till later, is of 

 special importance for vigorous assimilation, and hence its occur- 

 rence on the upper side of many leaves demands special attention. 



But even facts of comparative anatomy are of importance in 

 affording a basis for the view that the palisade parenchyma must 

 be regarded as a specific assimilatory tissue. 



Sarothamnus vulgaris is a shrub bearing only very small leaves, 

 which appear inadequate for the work of assimilation. Here the 

 much branched stem system must be operative as well as the 

 leaves, in order that sufficient quantities of organic substance 

 for the plant may be produced. A transverse section of the stem 

 presents the appearance of a five-rayed star, and if we examine 

 a thin section under the microscope, it will be found that under 

 the epidermis of the ends of the rays sclerenchymatous tissue is 

 present while the peripheral tissue of the intervening hollows 

 is green. We determine that this green tissue in its outer layers 

 consists of palisade cells, which are elongated at right angles to 

 the surface of the stems, while further inwards follow more 

 isodiametric chlorophyll-containing cells. We also examine a 

 thin transverse section of the stem of Spartium junceum. The 

 assimilatory tissue below the epidermis here consists entirely of 

 palisade cells. There are about six layers of much elongated 

 green cells arranged at right angles to the surface of the stem. 

 The green tissue does not however form a closed ring below the 

 skin, the assimilatory tissue alternating at the periphery through- 

 out the entire stem with sclerenchyma. In plants with poor 

 foliage, or plants which do not produce any green leaves at all, 

 the green tissue of the stem system must undertake the work of 



