February 2> 1885,] 



THE TROPICAL AGRICULTURIST. 



647 



of man. If any healthy tree be persistently defoliated 

 during the growing season, the tree will die. These facts 

 led early botanists to form the opinion that leaves were 

 merely " a clothing, or protection against colds and heats." 

 Though this ancient theory is certainly true to a certain 

 extent, still it is only a very small portion of the func- 

 tions of the leaves. For it is in the leaves that those 

 changes are effected by which the juices of the plant, 

 relieved from the water — which is unnecessary for the 

 functions of life— are rendered fitting for the nourishment 

 and growth of the plant. 



That leaves exhale moisture is proved also by the sim- 

 ple experiment of gathering the leafy branch of a tree, 

 and immediately stopping the wound at its base with 

 wax, or some fit substance, to prevent the effusion of 

 moisture in that direction. In a very short time the 

 leaves droop, wither, and fade. If the branch in this con- 

 dition be placed in a very damp cellar, or immersed in 

 water, the leaves revive and assume their original ap- 

 pearance. This experiment also shows their power of 

 absorption. 



The chief office of the stem appears to be to elevate 

 the leaves, the flowers and the fruit into the most favour- 

 able position for receiving the influence of light, heat and 

 air, for it is upon these that their full development de- 

 pends. But this is not its only function; in combination 

 with the roots and leaves (nature's wonderful laboratory) 

 the crude sap, in its ascent, is divested of its rawness 

 and aqueous matter and converted into nutritious sap, 

 capable not only of supplying to the different parts of 

 the structure the materials necessary for the maintenance 

 of their healthfalness, for the repair of injuries, and for 

 the production of entirely new parts, but also of furnish- 

 ing the ingredients of those several products which the 

 various tribes of plants may be said almost to create from 

 the elements around them, and which are so valuable to 

 man as articles of diet, as medicines or as articles of use 

 in his various manufactures. All these varied substances 

 originate in the ascending sap in its passage through the 

 sap wood (alburnum) and flowing sap (cambium layer) by 

 the exhaling process, in which the leaves play the most 

 conspicuous part, for by them it is concentrated by the 

 loss of its water not only into solid matter, but into those 

 secretions which almost every tribe of plauts produces 

 peculiar to itself. How remarkably these secretions of 

 different plants vary from each other, frequently in the 

 same plant ! The peach tree affords a familiar example 

 of this ; the gum of this tree is mild and mucilaginous ; 

 the bark, leaves and flowers abound with a bitter secre- 

 tion than which nothing can be more distinct from the 

 gum ; the fruit is replete not only with acid, mucilage, 

 and sugar, but with its own peculiar aromatic and highly 

 volatile secretion, on which its delicious flavor depeuds ; 

 the pit also contains prussic acid to a certain extent. Can 

 any one among the scientists tell us how the peach tree 

 can form, collect, and keep separate such a number of 

 distinct and discordant secretions? 



In all exogenous trees tho inuer and older portion is 

 much harder and dryer than the exterior. This change is 

 due to the consolidation of the interior wood, by the 

 deposition in its tissues of woody matter. The portion 

 of the stem in which this has taken place thus acquires 

 great toughness and durability, but it is no longer fit to 

 perform any office in the living system save that of me- 

 chanically supporting the rest, since no sap can pass in 

 any way through the now filled-up channels. Tbe heart 

 Wood always displays a deposit of coloring matter not 

 Bbscrvable in the younger or outside layers. Thus in the 

 wood of the aople or pear, tbe handsome orange color 

 of the heart wood marks the period of rest, and 

 as far as assisting growth is concerned, it is at rest for 

 evermore. It is through the newer layers or sap-wood, 

 therefore, that the larger portion of the sap ascends, and 

 these in their turn become inclosed by others, and arc at. 

 last consolidated like the more aged ones into heart wood. 

 The sap wool is enclosed by the bark, which, like it, 

 is formed in regular layers; though these are much thinner, 

 and cannot be so plainly distinguished. The layers of 

 bark are formed from the interior, so that the oldest 

 ones are on the outside. These are gradually lost, either 

 Way or by falling off; so that it is seldom that the 

 lame number of tinge can be ♦racej iu the bark as in 



the wood, although an additional one is formed each year. 

 As the new layer of wood is formed on the outside of 

 the previous one, at the point at which it was in con- 

 tact with the bark, and as the new layer of bark is 

 added to the inside of the previous one, at the point at 

 which it was in contact with the wood, it is obvious that 

 they are produced at the same spot, and that the 

 newest layers of both will always be in contact with 

 each other. 



Between these, always at the close of the season's 

 growth, will be found a glutinous fluid composed of gum 

 and sugar. The gummiuess of this fluid is at once per- 

 ceived by its mucilaginous properties, and that it contains 

 sugar is known by the sweetness of its taste. This is 

 the elaborated sap, ripened and prepared, after the wood 

 growth has been perfected, for being converted into an 

 organized tissue the succeeding year. This substance, 

 called the cambium layer, can be readily seen by strip- 

 ping the bark from almost any twig, in the early part of 

 spring. Later in the season this layer is gradually or- 

 ganized into cells, and from these are formed the ducts 

 and cellular portion of the season's woody layer. 



DOES SAP ASCEND. 



Fifty years ago, our text-books on vegetable physiology 

 taught us that the sap descended in the fall of the year 

 with as much facility as it ascended in the heat of 

 summer. To prove this theory, the following illustrations 

 were given : — "If a cord be tied tightly around the trunk 

 of an exoc/en, it offers little impediment to the ascent of 

 the sap, but will obstruct its diffusion through the bark 

 in its descent. In consequence, there will be a deficiency 

 of nourishment to the parts beneath, and a superfluity above; 

 so that a protuberance will arise from the stem just at 

 the point where the downward flow of the sap is checked. 

 This protuberance will increase in progress of years (if 

 the tree survive) so as to bury the cord beneath it ; but 

 most commonly the tree is destroyed ere long by an in. 

 sufficieut supply of nourishment." 



Again : It is in the cells and woody tubes of the alburnum 

 (sap wood) that the fluid absorbed by the roots is trans- 

 mitted to the opposite extremity of the stem, and these 

 cells communicate with those of the leaves, which receive 

 it from them. In the bark the woody tissue has a some- 

 what different form. Instead of the tubes (cells) lying 

 side by side (as in the sap wood), they branch horizont- 

 ally from each other, and their function is that of con- 

 veying back from the leaves the juices of the plant which 

 have been there elaborated out of the sap brought up 

 I by the vessels of the sap wood." Thus they explained 

 the theory called the descent of the sap. 



The study of the grand text-book of Nature for the past 

 fifty years has led us to believe that these theories, which 

 we have quoted, have no foundation in fact. We, who 

 are practical horticulturists, have often noticed those large 

 protuberances close to and over the union between the 

 scion snd the stalk when we grafted the puar on the 

 quince, the apple on the wild crab and the Heart varieties 

 of the cherry upon the Morello stalk, producing in a few 

 years larger protuborances than a cord would if tied tightly 

 around tho trunk of a healthy young exogou. Agaiu, the 

 amount of sap passing through the small cells of the bark 

 must be very small, compared with the amount that passes 

 through the sap wood and cambium layer, in a season 

 favorable to wood growth, when the tree adds one or two 

 inches of solid woody matter to its diameter. Every ob- 

 servant mind who has made this delightful science a close 

 study tu the grove or orchard, must have received ab 

 extra revelations from Nature, which are not imparted 

 to the scientist in his cabinet, and which the text-books 

 do not nor cannot give. 



Permit us to present a few illustrations in SuppOrl i I 

 the theory that the sap does not descend, but is elaborated, 

 retained, spread, ripened and compressed over the entire 

 inner surface of the tree during its upward flow. 



lu the year 1832 we had charge of a large conservatory 

 for the growth of palms, orchids, cacti, and other tropical 

 plants. In front, planted in a rich, artificial border, out- 

 side of the walls, grt"sv twelve grape vines of the Black 

 Hamburg variety, one vine opposite each rafter; age of 

 vines, twenty years; height of stems, three feet. At this 

 point they branched into two limbs, eaoh tbirtj fei 

 one limb, of each vine was introduced inaidl 



