40 



TLANT STUDIES 



air chambei's extends tlirougliout t]ie spongy mosopliyll. 

 It is into this system of air chaniljers tliat tlic stoniata 

 open, and so tliey are put into direct comnnniication with 

 tlie meso]iliyll or working cells. Ilie peeidiar ai'rungcnjent 

 of tlie njiper mesophyll, to form tlie jialisade tissue^ has to 

 do with the fact that tlia.t surface of the leaf is exposed to 

 the direct rays of light. This light, so necessary' to the 

 mesophyll, is also dangerous for at least two I'casons. If 



St St 



Fig. .'iO. A Hcctirm tlirougli tlu* leaf of lily, showing up]>er e])id(.rniis ('/<), lower epi- 

 (lenjiis (A J widi its stoniata (,';/), mcsopliyll (dotted cells) composed of the palisade 

 region i //i and the spongy region (s;j) with airspaces among the cells, and two 

 veins (r) cut across. 



the light is too intense it may destroy the chloroiihvll, and 

 the heat may di'y out the cells. P>y presenting only nar- 

 row ends to this direct light the cells are less exposed to 

 intense light and heat. Study Fig. ;>(). 



33. Veins. — In the (■ross-section of the Ictd' there will 

 also ho seen here adid there, emhedde(l in the meso|)hyll, 

 tlie cut ends of (he vcinlels, ma.dc n|) |i;irtl\- of thick- 

 walled cells, which hold (he leal' in sluipe and conduct 

 material to and from the mcsopliyll (see Fig, 3i.i). 



