60 ECOLOGY Part II 



ultimately continuous with the veins of the leaves. Stems vary in circumference: 

 the stem of a California redwood is thick enough for a car to drive through; 

 that of the young maidenhair fern has a hair's thickness. Stems are squat in 

 turnips and tall in royal palms. 



The main layers of the stem are the cambium, and the phloem, and xylem, 

 the latter two named from the Greek words for bark and wood. Cambium is 

 the vital growing layer from which the other two layers originate, the xylem 

 from its inner and the phloem from its outer side. In tree trunks the wood is 

 composed of xylem and most of the bark of phloem. The xylem holds the 

 supporting tissue and tubes through which water and dissolved substances are 

 conducted from root to leaf. The phloem contains tubes through which manu- 

 factured foods are distributed especially from the leaves to regions of the 

 plant where they are stored or used. The epidermis covers the stem and is 

 continuous over the leaves and roots. Tons of water mixed with mineral 

 nutrients ascend from the soil and through the tubes of the xylem into the 

 veins of the leaves. Great quantities of food made in the leaves pass through 

 the veins and stem by way of the tubes of the phloem. The pattern of con- 

 duction in xylem and phloem is essentially the same whether in a buttercup or 

 an oak tree. 



Sugar cane, potatoes which are underground stems, and tree trunks are 

 stems that have million-dollar values and high places in history. Except for the 

 plant stems that made his ships, Columbus would not have crossed the ocean 

 nor the Norsemen set foot upon American shores. A few plant stems made 

 the raft Kon-Tiki on which six men crossed the Pacific Ocean. 



Leaf. A leaf is a thin blade, greener on the upper than the underside and 

 freely exposed to light and air. Continuous with its petiole or stem is the 

 stiffened vein or group of veins from which other more delicate ones branch 

 olT and hold the leaf outspread. The unique function of green leaves is photo- 

 synthesis. Water from the plant stem is conducted to the leaf, and carbohydrate 

 food from the leaf to the plant stem. There is great variety in the shapes of 

 leaves, but, whether they are simple or compound they all fit three types: the 

 rounded leaf like that of the nasturtium, the linear leaf like the grass blade, 

 and the cone-shaped one such as the elm leaf. 



Microscopic openings of stomata occur in the otherwise waterproof epi- 

 dermis, especially on the lower side of the leaf (Fig. 4.8). Each opening is 

 between two specialized cells of the epidermis, called guard cells because 

 changes in their size and shape determine whether the stomata are open or 

 closed. Water enters through the root hairs and passes out mainly through the 

 open leaf-stomata and to some extent through the cuticle, in the process of 

 transpiration. Of the total quantity of water absorbed by the roots, as much as 

 98 per cent escapes by transpiration. Stomata also regulate the exchange of 

 gases between the air and leaf. If the leaf is well lighted they are open and 



