16 



THE PLANT: ITS STRUCTURE, LIFE -PROCESSES AND ENVIRONMENT 



which remain alive after the sieve cells are 

 apparently dead; their function is not clearly 

 understood. 



In dicotyledonous plants, between the wood and 

 the bast is found the cambium, an embryonic tissue 

 that forms new cells whose growth causes the stem 

 to thicken year by year. The inner part of this 

 growth becomes wood, which adds an "annual ring." 

 These rings are clearly marked, because the wood 

 formed in the fall is denser and has smaller cells 

 than that formed in the spring. The outer part of 

 the new growth becomes bast, which wears away 

 on the outside almost as fast as it forms within, 

 and, in consequence, does not thicken much from 

 year to year. Monocotyledonous stems have no 

 cambium and do not grow thicker from year to 

 year. 



The cambium causes the cion to unite to the 

 stock ; it heals wounds, such as are made by 

 pruning, by forming a tissue called callus. This 

 sometimes produces new buds, whose growth com- 

 pensates for the part cut away. At the tip of the 

 stem the cambium does not form a complete ring 

 but is confined to the fibro-vascular bundles. In 

 trees and shrubs it gradually extends itself from 

 one bundle to another, thus forming a complete 

 ring. As soon as this is accomplished, it begins to 

 form a complete ring of wood within and of bast 

 without. In herbs no such complete ring is formed. 

 Outside the bast is found the rind or cortex, 

 which is usually green, and, in consequence, manu- 

 factures starch. It also serves to convey starch. 

 This is easily seen when it is cut away all around 

 the tree, in the process of " ringing," whereupon the 

 tissues below lose their starch. If the bast be cut 

 through also, the supply of proteids is cut off and 

 death soon ensues. As the stem grows older, layers 

 of cork are formed in the rind. These cut off the tis- 

 sues lying outside them, which soon die and so form 

 bark. The very first layers of cork are formed on 

 the extreme outside of the stem, and are interrupted 

 at frequent intervals by 

 breathing pores or lenti- 

 cels. 



At the very tip of the 

 stem is found embryonic 

 tissue which continually 

 forms new cells ; this is 

 called the growing point. 

 Just back of this, new 

 leaves arise as protuber- 

 ances (Fig. 38). These 

 rapidly grow larger and 

 fold over in such a way 

 as to protect the growing 

 point from mechanical in- 

 juries as well as from dry- 

 ing. The various waxes, 

 resins and furry coverings 

 of buds are not for protection against cold, as popu- 

 larly supposed, but for protection against drying. 

 The crowding of the young leaves at the grow- 

 ing point, forces them to take on a regular ar- 

 rangement which largely determines the arrange- 

 ment of the branches, since these arise from the 



Fig. 38. Bud Of brusaels 

 sprouts cut lengthwise. 

 f, fibrous bundles; &^ 

 the crumpled leaf- 

 blade. 



Fig. 39. 

 Diagram showing the gir- 

 der-like arrangement 

 of strengthening tissues 

 l.etr) in a bulrush. 

 Scirpus. 



point (axil) where the leaf is joined to the stem. 

 Not all branches develop. Many that start cannot 

 get sufficient light and soon die. This is known as 

 " self-pruning," and is seen especially in forest trees, 

 which produce lumber free from knots. Many buds 

 do not even start to de- 

 velop but remain dormant, 

 often for many years, 

 growing just enough to 

 keep pace with the annual 

 thickening of the tree. 

 They may be traced back 

 to the center of the tree, 

 sometimes several feet 

 long, but no thicker than a 

 lead-pencil. New or adven- 

 titious buds may be formed; 

 such buds, becoming 

 crowded and distorting the 

 grain of the wood, cause the appearance familiar 

 in bird's-eye maple. 



The stem requires strengthening tissue in order 

 to sustain the weight of its branches and the force 

 of the wind. In the tree the accumulated wood 

 serves every purpose, but in the herbaceous stem 

 special strengthening tissue is formed, quite dis- 

 tinct from the wood. In parts of the stem that 

 are lengthening, this tissue consists of collenchyma 

 cells, whose walls, thickened at the corners only, have 

 thin places by means of which food and water may 

 be absorbed (Fig. 24). Their growth keeps paca 

 with that of the stem, otherwise they would soon 

 break and become useless. In older parts of the 

 stem that have ceased lengthening, the mechanical 

 cells, sclerenchyma, have walls equally thickened 

 all around, except at the pits ; when the thickening 

 reaches a certain point the cells die, but their use- 

 fulness is not impaired thereby. 



The distribution of mechanical tissue in the stem 

 presents a wonderful example of useful arrange- 

 ment to secure the highest degree of strength 

 with the least expenditure of material. The prin- 

 ciple of the girder and of the hollow cylinder is 

 everywhere employed (Fig. 39) in leaf and stem. 

 It results that a wheat stalk is a model of light- 

 ness and strength, and at the same time it is elas- 

 tic enough to bend sufficiently in the wind. In the 

 root (Fig. 29) the strengthening tissue forms 

 strands in the center, known as cable construction, 

 that enable it to resist pulling strains. Some 

 stems economize material by climbing on walls, 

 trees, or other supports. Some weave themselves 

 in and out of the branches of other plants (black- 

 berry), others form tendrils by modifying a branch 

 (squash, grape), or a leaf (pea), or a leaf-stalk 

 (clematis). The coiling of the tendril is due to 

 a stimulus such that the contact side grows less 

 rapidly than the opposite side. The tendril, and 

 the tip of the stem as well, usually has a sweeping 

 circular movement that assists in finding a sup- 

 port. The tendrils of the Boston ivy fasten them- 

 selves to walls ; the roots of the English ivy 

 answer the same purpose. 



Plants which twine do so apparently under the 

 influence of gravity, which causes one side to grow 



