September 29, 1906 



HOKTICULTURi: 



337 



Plant Tissues 



111 \]L'\v of Mr. L'haudltr'.- recent observations on the 

 growth of cambium tissue, in Horticulture, Sept. 8, 

 1906, tlie following notes on plant tissues may be of 

 interest. 



All our common higher plants are composed of in- 

 numerable small cells. Tlu'se cells differ very widely 

 from each other in shape, size and texture according to 

 the work they do. Any group of similar cells is called 

 a tissue. In a tree, such as the silver maple of Mr. 

 Chandler's experiment the various tissues may be 

 classified according to function into five groups. 



1. Formative tissues. These are composed of very 

 small thin walled cells which are meristematic, i. e., 

 capable of active division and growth and which give 

 rise to all the other tissues in the plant. The cambium 

 tissue is one type of formative tissue. I shall speak of 

 these meristematic tissues in detail but will first men- 

 tion the remaining functional types of plant tissues. 



",.'. Protective tissues. These are of two kinds : 

 first, tegumentary or external including epidermal tis- 

 sue (epidermis, hairs and scales), cork and bark. The 

 epidermis of young twigs is soon thrown off and is re- 

 placed by the dull, gray cast cells forming the outer 

 bark of older twigs and limbs. Second, mechanical, 

 usually internal and always dead tissue when perform- 

 ing its natural function. These are well illustrated by 

 the surrounding sheatli of the tough threads in corn 

 pith. Each such thread is a bundle of tubes for con- 

 ducting sap up and down the stem. The bundle is 

 securely supported on the outside by long, thick walled 

 fibre cells. These strengthening cells are pointed at the 

 tip to make end contact stronger and in some plants 

 liave swellings and irregularities on their side walls to 

 ilovetail the cells together into a firm, solid tissue. 



The fibres do not surround the bundle uniformly but 

 are grouped mainly in two regions. This arrangement 

 is a direct adaptation to function. The fibres form two 

 broad bands, one on the side next the centre of the 

 stalk and the other in the opposite position. The lat- 

 eral or radial sides of the bundle have few thick walled 

 cells. There is thus formed a girder structure with the 

 strongest tissues where the swaying of the stalk would 

 throw the greatest stress upon the bundles. The other 

 conductive cells forming the body of the bundle serve 

 to tie the two strengthening areas together just as the 

 narrow piece of steel connects the heavy upper and 

 lower surfaces of a car rail. The absence of many thick 

 walled cells on the radial sides of the bundle permits of 

 free passage of liquids to and from the tissues through 

 which the bundle goes. The girder, hollow cylinder and 

 other fundamental structures of architectural design are 

 found to be abundant and complex in the strengthening 

 tissue of plants. 



3. Nutritive tissues. Tln-e tissues are always liv- 

 ing cells while performing (lieir natural function and 

 include the following types: 



a. Absorbing tissues, the root hairs through which 

 the soil solutions are t<iken into the feeding rootlets. 



b. Assimilating tissues, cells located in the leaves 

 and containing the green chlorophyll bodies which 

 aljsorb energA' from tlie sunlight and thus assimilate 

 carbon from the air -aiu] build it into starch compounds 

 for plant food. 



c. Conducting tissues, the veins of leaves and the 

 fibrovascular bundles of stems as the corn fibres already 

 referred to. Some of these duets are long tubes 

 foriiud by cells connected end to end from which the 

 cross partitions have largely disappeared. Through 

 such modified cells liquids pass readily to all parts of 

 the jilant. 



il. Food -toring tissues. The epidermal celLs store 

 up water. They are found to be quite empty in time of 

 drought and full when the water supply is large. The 

 wdoil (ills of many winter twigs are well filled with 

 starch, particularly in the early blooming plants. 

 Tuliers and l)ulbs are store houses for plant food and 

 even small seeds eoiituin tissues which are filled with 

 reserve food material. 



e. Arcating tissues, the elaborate stomate structures 

 or breathing pores of the leaves. 



f. Secreting and excreting tissues. Examples of 

 these are water glands, resin glands, nectaries secreting 

 the honey dew of flowers and crystal bearing cells. The 

 most common crystal in plant tissues is calcium oxalate. 

 These crystals are excreted to rid the plant of extra 

 calciums. 



4. Special tissues and apparatus for particular 

 duties such as holdfasts, parachutes, floating devices, 

 organs for receiving and conducting stimule and h3'gro- 

 scopic tissue which in drying serves to open fruits. 



5. Keproductive tissues. These are the outer 

 flower parts, sepals, petals, stamens and pistil which are 

 leaves variously modified to aid the plant in the pro- 

 duction of seed. The real reproductive agents, the 

 pollen cells and the ovules which produce the male 

 nuclei and the egg cells are active meristematic cells 

 and would fall in the category of formative tissues. 

 When a male nucleus fuses with an egg cell nucleus the 

 egg cell resumes meristematic activity and will devel- 

 op an embryo plant. By the union of such repro- 

 ductive elements the characteristics of the parent plants 

 are transmitted to the offspring. The egg cell now 

 becomes a germ cell whose protoplasmic contents is uni- 

 formly cliarged with the possibilities of the parent 

 plants. Environment does things to this living tissue 

 and it responds in certain ways as its inherited proper- 

 ties enable it. Such an embryo ])lant witii its .stored 

 food supply and protective coverings is called a seed. 



With this brief explanation of jilant tissues we will 

 return to the diseussicm of formative tissues and how 

 a tree grows in height and diameter. 



:/rm/^ 



Park Dept., Bosion, Mass. 



(To be continued.) 



