LEAF AND STEM STRUCTUKE OF THE OLIVE. 



51 



Fig. 18.— Ventral face of an olive leaf, showing the 

 thickened waUs of epidermal cells and palisade cells, 

 (X 150.) 



numerous needle-shaped crystals of calcium oxalate, are of a very 

 irregular shape and the intercellular spaces are very wide (fig. 19). 

 The pneumatic tissue, like the palisade tissue, is broken at the midrib 

 by hypodermal collenchyma. 



The stereome is thick walled and very unequally distributed. It 

 occurs hypodermally (immediately beneath the epidermis) as single 

 cells or a few cells together on 

 both faces of the blade (fig. 18), 

 as scattered cells in the col- 

 lenchyma (PL V, fig. 1), and as 

 a pericycle of several continuous 

 layers in the midrib (PI. V, fig. 

 1). It is characteristic of the 

 genus Olea that the stereome 

 cells traverse the pneumatic 

 tissue in all directions (fig. 19). 

 The pericylic stereome is thick 

 walled only on the hadrome side 

 of the midrib; on the leptome 

 side it is thin walled with a very 



few thick-walled cells interspersed. The collenchyma (PI. V, fig. 1) is 

 hypodermal above and below the midrib and extends to the pericycle; 

 it is generally thick walled, especially near the epidermis. 



The mestome strands are, with the exception of the midrib (PI. V, 

 fig. 1), embedded in the chlorenchyma, and all the lateral strands 

 are surrounded by thin-walled parenchyma sheaths, sometimes with 



a few adjoining stereome cells. 

 The midrib has no parenchyma 

 sheath and no endodermis, but, as 

 previously described, it is surround- 

 ed by a thick sheath of stereome. 

 All the mestome strands are col- 

 lateral. The leptome forms an 

 arch underneath the shorter but 

 broader arch of hadrome. In the 

 latter, each double row of vessels is 

 separated from the next by a single 

 row of parenchyma cells (parencliy- 

 matic ray). 

 The petiole, examined at the characteristic point (where the mes- 

 tome strands enter the leaf blade), shows a hemicylindric outline in 

 cross section. It is covered with shield-shaped hairs, as is the blade, 

 and the outer walls of the epidermis cells are extremely thick. The 

 cortex is a solid mass of collenchymatic tissue and contains an arch- 

 shaped collateral mestome strand in the center. This mestome 



192 X 



FiG. 19.— Pneumatic tissue of the dorsal side of a 

 blade traversed by stereome cells. From a leaf 

 of the Mission olive. (X 150.) 



