THE AXGIOSPERMAE 



1485 



and their lower ends may be suberized. The cells are generally closely 

 packed, but in some seeds there are irregularities of diameter which produce 



Fig 



B 



1358. — Canna indica. A, Section 

 through the testa showing the marked 

 pahsade layer, derived from the 

 epidermis. The position of the " hght 

 hne " is indicated by the dotted hnes. 

 Below is a zone of stone cells and the 

 crushed remains of the inner integu- 

 ment, abutting on the endosperm. 

 B, A single palisade cell showing 

 the contracted cell lumen. {After 

 Humphrey.) 



narrow^ intercellular spaces, while in Nehimbo and in Catnia the palisade 

 is traversed bv stomatal canals. 



Prominent in nearly ever}^ case is a narrow, almost colourless band, 

 running tangentially through the whole palisade, near the top of the cells. 

 This is called the " light line " or " light zone " and it has attracted much 

 attention, based on the supposition that, like the Casparian band around 

 endodermal cells, it constituted an impermeable, sealing belt around the 

 palisade cells, which prevented the entry of water unless it were first 

 ruptured. There seems to be little evidence for such an idea, for the zone 

 does not appear to be chemically different from the rest of the cell wall, 

 although in 7V//« it is reported to be lignified. The most careful work in- 

 dicates that it is only an optical effect, due either to a convex enlargement of 

 the cell wall at that level, or to the refraction of light at a surface of 

 contact between two transparent substances of different refractive index 

 (the " Becke Line"). The latter certainly seems to be true in Melilotiis, 

 examined by Hambly, where the light line is formed at the junction 

 between the suberized caps of the cells and the cellulosic body. This 

 worker showed that when the microscope objective is raised the line 

 appears to move towards the substance with the higher refractive index, as 

 the Becke hne does. This is probably not a universal explanation, for in 



