3° 



DIFFERENTIATION OF THE TISSUES 



are filled with cell-sap, largely water, and serve in many cases 

 as an effective storehouse for water. This function is carried 

 to its fullest development where a multiple epidermis is produced 

 by the tangential division of the protoderm, as in the rubber 

 leaf (Fig. 13). 



In herbaceous plants, and in leaves generally and fleshy fruits 

 the epidermis remains, but in the perennial parts of plants that 



increase in size from year to year, such as 

 the stems and roots of trees and shrubs, and 

 in many underground parts that endure but 

 for a season, such as the Irish potato, cork 

 is after a time produced beneath the epi- 

 dermis and this dies and sloughs away. 



Primary Cortex. — All of the ground 

 meristem lying exterior to the procambium 

 strands (which, it will be remembered later 

 become the primary vascular bundles) 

 produces permanent tissues that are divisi- 

 ble into two zones by a layer of cells, more 

 or less continuous, knowm as the starch 

 sheath or endodermis. The outer of these 

 zones, which includes the starch sheath, is 

 known as the primary cortex (Fig. 14). 

 (The term primary cortex must not be 

 confused with cortex as used in pharmacognosy, where cortex, 

 employed synonymously with bark, is often applied to all of 

 the tissues collectively which lie exterior to the cambium ring.) 

 The primary cortex does not as a rule consist of a single tissue, 

 but of two or more, so that in describing its evolution from 

 the ground meristem the possible tissues composing it must be 

 considered separately. 



Beginning at its exterior just beneath the epidermis we com- 

 monly find a tissue whose walls are thickened at the angles where 

 three or four cells join. This tissue is called the collenchyma 

 (Fig. 14). It is one of the first of the primary tissues to come 

 to maturity, and its chief function, in virtue of its thickened 



Fig. 13. — Cross section 

 of a portion of leaf of Ficus 

 elastica showing the mul- 

 tiple epidermis from e to a 

 inclusive; c, cystolith; b, 

 palisade parenchyma; d, 

 spongy parenchyma. 

 (After Sachs.) 



