Elementary botany . fthe juniper, arbor vitae, cypress, cassiope, pyxidanthera, etc., thatthere has been a reduction in the size of the leaf, and at thesame time an increase in thickness. This with the crowdingtogether of the leaves and their thick cell-walls greatly lessensthe radiation of moisture and heat, thus protecting the leavesboth in dry and cold weather. The succulents, like live-for-ever, have a small amount of surface in proportion to the massof the leaf. In the yucca, though the leaves are often large,they are very thick and expose a comparatively small amountof surface to the dry
Elementary botany . fthe juniper, arbor vitae, cypress, cassiope, pyxidanthera, etc., thatthere has been a reduction in the size of the leaf, and at thesame time an increase in thickness. This with the crowdingtogether of the leaves and their thick cell-walls greatly lessensthe radiation of moisture and heat, thus protecting the leavesboth in dry and cold weather. The succulents, like live-for-ever, have a small amount of surface in proportion to the massof the leaf. In the yucca, though the leaves are often large,they are very thick and expose a comparatively small amountof surface to the dry air and intense sunlight of the desert epidermal covering is also hard and thick. In addition. FOLIAGE LEAVES. 395 such leaves, as well as those of many succulents, are so thickthey provide water storage sufficient for the plants, which radi-ate so slowly from their surface. 766. (4) Elimination of the leaf.—Perhaps the most strikingillustration of the reduction of leaf surface is in those cases where. Fig. Phylloclade, leaves absent, stems broadened to function as leaves, on theedges numerous flowers are borne. the leaf is either completely eliminated as in certain euphorbias,or in certain of the cacti where the leaves are thought to be re-duced to spines. Whether the cactus spine belongs to the leafseries or not, the leaf as an organ for assimilation and trans-piration has been completely eliminated and the same is truein the phylloclades. The leaf function has been assumed bythe stem. The stem in this case contains all the chlorophyll;is bulky, and provides water storage. III. Protective Positions. 767. In many cases the leaves are arranged either in relationto the stem, or to each other, or to the ground, in such a wayas to give protection from too great radiation of heat or the examples already cited the imbricated leaves of cassiope, 39^ DELATION TO ENVIRONMENT pyxidanthera, juniper, etc., come also under this head. In thejunipers the leav
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