TISSUES OF NUTRITION. 



135 



Fig. 46.— Leaf of Dieitalis purpurea showing 

 veins or conducting vessels. (Planchon). 



ed fashion, as in the Dicotyledons. (Fig. 

 46.) Between these fibro-vtiscular bun- 

 dles the fundamental tissue of the leaf 

 the parenchyma or mesophyll, is ar- 

 ranged in various ways. Surrounding 

 the whole surface is the epidermis, which 

 is a direct continuation of the epidermis 

 of the stem. • . 



Thus, each leaf has portions of at 

 least three distinct types of tissue: The 

 Epidermal tissues, with its modifications, 

 stomata and trichomes; the Conducting 

 tissues, with, perhaps, some mechanical 

 support; veins, and the spongy Internal 

 tissue of the leaf proper, namely, the 

 Mesophyll, which makes up the tissues 

 of Assimilation, (Fig. 47.) 



The Mesophyll alone will here be de- 

 scribed, leaving the veins to be discussed 

 under Conduction System and the Sto- 

 mata with the Respiratory tissues. 



The Mesopliyll consists of thin-walled 

 parenchymatic cells, which are general- 

 ly loosely arranged, giving to it the name 

 of Spongy Tissue. In some leaves there 

 is a single or double row of cells, which 

 are somewhat cylindrical or brick-shaped 

 and which are placed with their longest 

 diameters at right angles to the epider- 

 mis. They are thin walled and filled 

 with chlorophyll. They are called Palis- 

 ade Cells, and by means of their 

 chlorophyll assimilation of the carbon di- 

 oxide of the air is carried on. 



According to the arrangement of the 

 palisade cells three types of leaves have 

 been described. 



1, Leaves without any palisade cells. 



Centric, 



2, Leaves with palisade cells on both 



surfaces. Iso-lateral. 



3, Leaves with palisade cells on the 

 upper side only. Bi-facial. 



To the first belong many herbs which 

 grow in the shade, also many stipules 

 and calyx lobes, the latter having little 

 or no functions of assimilation. 



In the Iso-lateral type a single or 

 double layer of palisade cells is found, 

 with a spongy mass of parenchymatic 

 cells between forming a middle layer. 

 In general leaves that are affected on 

 both sides equally by the sun have this 

 internal arrangement, hence it is found 

 more often in erect monocotyledonous 

 leaves, as in Aloes. It is also found in 

 Senna, Eucalyptus and Lactuca. (Fig. 



48.) 



The third type, Bi-facial, is the more 

 common. It is found in most leaves 

 which are horizontally placed with ref- 

 erence to the sun*s rays. Jaborandi, Ab- 

 sinthium, Conium. Mentha piperita. Lau- 

 rel, Matico, Uva Trsi, etc., in which the 

 palisade cells may be ^ne, two or five 

 rows in thickness. 



It is an easy matter to determine xuost 

 of the official leaves by means of their 

 external shape and internal arrange- 

 ment, but in a powdered condition the 

 determination becomes very difficult, and 

 the epidermal cells, the trichomes and 



