. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. desert re-gions and in places where plantsat certain seasons are in danger ofsuffering from scarcity of water ithas been found advantageous to bic. 66.—A, depressed stoma of plants tO equip the StOmatal ap- the under side of a leaf of Amherstia.,11. ,1 i nobilis. B, depressed stoma of paratus with devices that retard Hakea suaveolens. g< strands be_evaporation while allowing the neath the guard ceils; d, outer, and e inner, cavities. (After Haberlandt.) diffusion of gases into
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. desert re-gions and in places where plantsat certain seasons are in danger ofsuffering from scarcity of water ithas been found advantageous to bic. 66.—A, depressed stoma of plants tO equip the StOmatal ap- the under side of a leaf of Amherstia.,11. ,1 i nobilis. B, depressed stoma of paratus with devices that retard Hakea suaveolens. g< strands be_evaporation while allowing the neath the guard ceils; d, outer, and e inner, cavities. (After Haberlandt.) diffusion of gases into the leaf. A common plan is to sink the stomata below the level of thegeneral epidermis, so that each is at the bottom of a pit wherethe wind scannot sweep away at once the water vapor asfast as it is transpired. Sometimes the outer wall of theguard cells is elevated in the form of a crater, or an elevatedborder of wax may serve to maintain a quiet atmosphere justabove the stoma. However the pit or crater is produced it issometimes made more effective by outgrowths that partially roofit over, as in Fig. I^O INTAKE AND DISTRIBUTION OF GASES \J The Lenticels.—In stems where the epidermis is being re-placed by cork the phellogen or cork cambium (see page 54)instead of producing ordinary cork tissue immediately belowthe stomata forms loose layers of cells, through the intercell-ular spaces of which gases pass in and out (Fig. 22). Thesegroups of loose cells are called lenticels. As the lenticels be-come larger by additions from the phellogen they press outand burst the epidermis and appear at the surface as rounded,elliptical or oblong excrescences. The formation of lenticels outruns that of the cork tissue, sothat when the cork has encased the stem and replaced the epi-dermis as a protective tissue the lenticels have taken the placeof the stomata and have provided aeriferous channels fromthe exterior entirely through the periderm. (See page 55.) The intercellular spaces
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