November 24, 1906 



HORTlCUl^TURi: 



553 



Plant Tissues 



Part IV. 



In tins last article I shall discuss secondary thicken- 

 ing or increase in diameter from year to year, explain 

 bark development and cork formation and conclude with 

 some recently published records of bud growth in fruit 

 trees. 



Secondary thickening is now very easily explained. 

 The condition of things in a one-year-old twig at the 

 close of the growing season was described in the second 

 article. I did not mention there, however, that the 

 cambium soon begins to grow out from the sides of 

 each of the concentrically arranged bundles until it 

 meets across the pith rays and forms a continuous 

 cylinder of cambium tissue; this extends down around 

 the twig from the formative cap at the growing point 

 like the finger of a glove, separating the wood elements 

 (xylem, pithray cells and pitli) within from the bark 

 elements (phloem, outer ends of pithray cells, cortex 

 and epidermis) without. This is the usual origin of the 

 continuous cambium layer. In some cases, however, it 

 may be developed uniformly at the growing point from 

 a ring of procambium tissue instead of as radiating 

 strands which later grow together. 



Each spring the cambium forms a new layer of 

 xylem or wood including pithrays around the old wood 

 and a new layer of phloem or inner bark beneath the old 

 bark. This annual layer is continuous over the wliole 

 system, both trunk roots, and limbs and all living twigs 

 and branches. The thickness of the ring varies 

 with the season, the age of the tree and the location on 

 the plant. One-twelfth of an inch represents a good 

 thrifty growth. This decreases as the tree grows older 

 and is thickest on the side of the trunk best supplied 

 with limbs and branches. Excessive growth, such as 

 one-half inch a season, is seen only in the central or 

 first rings of very thrifty trees. The ready peeling of 

 bark in the spring is due to the presence of tlie soft, 

 actively growing cambium tissue. It is the cambium 

 which unites the eion and stock, which heals the 

 wounds in plant stems and which dies when a tree is 

 girdled. 



There is no difference between the elements of first 

 year bundles and later bundles except that the first 

 large vessels of later years' growth are now beyond the 

 region of elongation and therefore their walls are pitted 

 and not thickened only in spirals or bands. Spiral or 

 annular vessels are found only in the primary wood 

 next the pith. The same contrast is present, however, 

 between the first and last formed wood elements in each 

 subsequent year's growth the large conducting tissue 

 is developed early and the smaller strengthening tissue 

 later in the season. It is this condition which gives 

 rise to the terms spring and summer wood. This con- 

 trast also makes the yearly rings of growth, i. e., it 

 marks where the small summer wood of one season 

 abuts against the large spring wood of the next. The 

 term sap wood applies to the lighter colored zone of 



wood extending two or llirce inches inward from the 

 cambium tissue and containing the thirty, forty, fifty 

 or more last formed yearly rings or growth. The cells 

 of the sap wood couduct water and food solutions and 

 otherwise assist in the life processes of the tree. The 

 inner darker part of the trunk is the heart wood. The 

 cells of this tissue are lifeless and their walls infiltrated 

 with chemical substances. The heart wood functions 

 simply as a mechanical support for the tree. 



As the branch increases in circumference the cam- 

 bium layer must keep, pace with it by lateral, i. e., 

 tangential growth and by the insertion of other bundles 

 with secondary pithrays between them. The epidermis 

 of young twigs is soon shed and the cortex is also lost 

 in a few seasons. The whole is replaced by cork cells, 

 strengthening-tissue and the outer phloem or bast cells 

 which have become crushed and useless as proteid con- 

 ductors. This mixture forms the outer bark of older 

 twigs and limbs. The winter value of such plants as 

 Cornus stolonifera and Kerria Japonica is due to the 

 brightly colored epidermis of present season twigs. If 

 such plants are cut back severely early in spring they 

 will make vigorous growth of new wood and produce 

 tine masses of color in the winter time. 



The formation of cork tissue is characteristic of the 

 stems of woody plants. It begins at different ages in 

 different plants. The cork cambium, or phellogen as it 

 is called, is quite similar to the cambium tissue of the 

 stem, except that the cork cells are usually developed 

 only on the outer side, the inner part remaining as 

 phellogen. Usually the phellogen is produced uniform- 

 ly all around the stem and sometimes it is localized as 

 in Euonymus alatus, where it is abnormally developed 

 in four longitudinal strands giving rise to the four 

 wings on the stem. The epidermis is first crowded off 

 at the stomates by the growth of the loose mass of cork 

 cells. The result is large ruptures which form the 

 lenticels so conspicuous in birch, cherry, alder, etc. The 

 cork cells give the gray color to older twigs and function 

 to retard transpiration and allow increase in size by 

 their power of elasticity. As the tree grows older the 

 bark on the trunk and limbs breaks and cracks, grad- 

 ually weathering off and producing the peculiar fissures 

 and patterns characteristic of (lie Itark of the different 

 kinds of trees. 



In this discussion of plant tissues nothing has been 

 said as to what time growth begins in the spring, when 

 it reaches the maximum or how long it lasts. In fact 

 little is known concerning the actual time and amount 

 of growth in plant tissues. The meagreness of our in- 

 formation shows the need of such experiments and 

 makes observations like Mr. Chandler's in Horticdl- 

 TUKE, September 8, '06, very interesting. In view of 

 this it will also be of interest to Icnow of the publication 

 of some careful records of bud growth in fruit trees. I 

 refer to the paper on "The Biology of Winter Buds," 



