ORGANS AND TISSUES. 



11 



FIG. 3. (After Sachs.) Cross-section 

 through a group of dead, thick- 

 walled wood-cells from the stem of 

 maize. The cells contain only air or 

 water. (Highly magnified.) 



plant fast and absorb various substances from the soil ; the stem 

 supports the leaves and flowers 

 and conducts the sap ; the leaves 

 absorb and elaborate portions of 

 the food ; and the reproductive 

 organs of the flower serve to 

 form and bring to maturity seeds 

 destined to give rise to a new gen- 

 eration. 



Heterogeneity of the kind 



o / 



just indicated, accompanied by a 

 division of labor among the 

 parts, is one of the most char- 

 acteristic features of living things, 

 and is not known in any mass of 

 lifeless matter, however large and 



o 



complex. 



Organs composed of Tissues. Differentiation. In the next 

 place, it is to be observed that the organs also, when fully 

 formed, are not homogeneous, but are in turn made up of 

 different parts. The human hand is an organ which consists 

 of many parts, differing widely in structure and function. On 

 the outside are the skin, the hairs, the nails ; inside are bones, 

 muscles, tendons, ligaments, blood-vessels, and nerves. The leaf 

 of a plant is an organ consisting of a woody framework (the 

 u veins 51 ) which supports a green pulp, the whole being covered 

 on the outside by a delicate transparent skin. In like manner 

 every organ of the higher plants or animals may be resolved 

 into different parts, and these are known as tissues. The 

 tissues of fully formed organs are often very different from one 

 another, as in the cases just mentioned ; that is, they are well 

 differentiated; but frequently in adult organs, and always in those 

 which are sufficiently young, the tissues shade gradually into 

 one another, so that no definite line can be drawn between them. 

 In such cases they are said to be less differentiated. For ex- 

 ample, in the full-grown leaf of a plant the woody framework, the 

 green cells, and the skin exist as three plainly different tissues. 

 But in younger leaves these same tissues are less different, and 

 in very young leaves, still in the bud, there are no visible differ- 



