SUPPLEMENT 



densely umbrageous trees, such as are characteristic of humid coun- 

 tries like England, are not found at all, but there is a marked con- 

 trast between the Eucalyptus, live oak and pepper on the one hand, 

 and the sycamore, maple, walnut and umbrella-tree on the other. 



A fuller study of the stems of trees than has been suggested in the 

 Reader, could doubtless be carried on with much interest, and profit, 

 especially in tire upper grades. All text books of Botany treat ot 

 stem structure rather fully, and the more recent ones, such as Spald- 

 ing's and Bergen 's, include the physiology of the stem also. From 

 twigs of various ages, and from pieces of firewood, one can, without 

 the microscope, make out layers of bark, the cambium layer, wood, 

 pith, medullary rays, and rings of annular growth. Very beautiful 

 wood sections, prepared by R. B. Hough, Lowville, N. Y., are often 

 included in school supplies ; Mr. Hough has recently added sections 

 of many California woods to his stock. These sections show clearly 

 to the unaided eye, the medullary rays, rings of annual growth, and 

 even to some extent, cellular structure. For the details of cellular 

 structure the compound microscope is of course necessary. The out- 

 side skin, or epidermis, of stems perishes early, and the layer of 

 green cells just beneath the corky layer also disappears in early years. 

 The corky layer, so greatly developed in cork oak, is protective in 

 several ways ; against transpiration, changes of temperature, attacks 

 of fungi, mechanical injuries, and so forth. It contains the same 

 tallow-like substance, suberin, as the cuticle of mature leaves. The 

 inner layer of bark consists of bast tissue ; it contains long, tough 

 cells, which serve for strength only, and more delicate ones, called 

 sieve cells. The soft bast tissue plays a very important part in the 

 nutrition of the plant, for it conducts the elaborated sap from the 

 leaves toward the roots, the medullary rays serving to convey the 

 sap laterally from bark to pith The woody tissue is made up of wood cells 

 and wood ducts, or vessels, as shown in Fig. 10, Chapter II ; it serves for 

 support and for the conduction of crude sap from roots to leaves. 

 The pith, which forms a main part of the young stem, usually dis- 

 appears in time ; where it exists, it conducts elaborated sap and 

 serves to store food during the dormant period, the soft bast and 

 medullary rays also serving the same purpose. The amount of food 

 stored in stems above ground is very considerable. In the trunks of 

 old, yet vigorous trees, a considerable part of the wood, as well as the 

 pith, is dead ; the outer part of the bark, too, is constantly wearing 

 away, sometimes falling in plates or shreds ; but in every active tree 

 there must be living wood to conduct crude sap from roots to leaves, 

 and living bark to distribute elaborated sap. Between these living 



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