42 CHAPTER 3 Cells, Tissues, and Organs: The Microscopic Components of Plant Structure © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR Cells,SALE OR DISTRIBUTION Tissues, andNOT FOR SALE OR DISTRIBUTION Organs: The Microscopic © Jones & Bartlett Learning,Components LLC © Jones of & Bartlett Plant Learning, LLC 3NOT FOR SALE OR DISTRIBUTIONStructure NOT FOR SALE OR DISTRIBUTION © JonesPlants & are Bartlett composed Learning, of a variety LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION ll organisms containNOT a FORmaterial SALE called OR pr otoplasmDISTRIBUTION, which is composed of of cells and tissues that must water, numerous organic compounds, and mineral nutrients. The nature work together in a coordinated of protoplasm was not well understood until the early 1900s. For hundreds fashion if the plants are to A survive. These are cells that of years it was thought to be a unique substance with an extraordinary property not conduct water (red) and sugars found in other substances such as rock, water, air, iron, and so on: protoplasm could (blue) in the veins of a leaf. © Jonesbe alive. & Bartlett One of theLearning, first experiments LLC to actually analyze ©protoplasm Jones & wasBartlett reported Learning, in LLC NOT FOR1644 SALEby Jean-Baptiste OR DISTRIBUTION van Helmont. He carefully weighedNOT a small FOR willow SALE plant OR and DISTRIBUTION a pot of soil. After planting the willow in the soil, he cultivated it for 5 years, then unpotted it, washed the roots to obtain all remaining soil, and discovered that the plant had increased in weight by 164 pounds (73 kg [kilograms]) and the soil had © Jones & Bartlett lostLearning, only 2 ounces LLC (57 g [grams]). He concluded© Jones that & Bartlett the plant Learning,and its protoplasm LLC NOT FOR SALE ORmust DISTRIBUTION be composed mostly of altered waterNOT and FOR a small SALE amount OR of DISTRIBUTIONminerals. He con- cluded that protoplasm might be special, but it was composed of ordinary chemical compounds. Other chemists confirmed that protoplasm is indeed composed of water, organic compounds, and mineral nutrients, but the belief still persisted that protoplasm © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION. Cells, Tissues, and Organs: The Microscopic Components of Plant Structure CHAPTER 3 43 © Jones must& Bartlett differ from Learning, other substances LLC by having a component© Jones called & “vital Bartlett force,” Learning, which LLC NOT FORif SALEpresent ORallowed DISTRIBUTION it to be alive and if absent causedNOT it to beFOR dead. SALE Only ORin the DISTRIBUTION late 1800s did Louis Pasteur and others prove that vital force does not exist: protoplasm is alive when its many complex chemical reactions are running properly, it dies if some of these reactions fail. The protoplasm of© all Jones organisms & Bartlett is divided Learning, into small bodiesLLC called cells, but our © Jones & Bartlett Learning, LLC understanding of cellsNOT was FORalso slow SALE to dev ORelop. DISTRIBUTION Early Greek philosophers knew that NOT FOR SALE OR DISTRIBUTION living creatures have organs: plants have roots, stems, and leaves; animals have hearts, stomachs, and so on. But even with the best eyesight, organs looked merely homoge- neous or at most mealy or fibrous. After the first microscopes were invented, in 1665 Robert Hooke discovered that plant organs are composed of small boxes, which he named© Jonescells. Plant & Bartlettcells were Learning, easy to study LLC even with crude early microscopes© Jones & be- Bartlett Learning, LLC causeNOT plant FORcells are SALE large ORand eachDISTRIBUTION is surrounded by a cell wall that appearsNOT FOR as a fine SALE OR DISTRIBUTION line when viewed by a microscope. It was quickly realized that all parts of a plant’s body are composed of cells. Also, early plant microscopists realized that plant cells are composed of small units (organelles): all green cells in leaves, stems, and unripe © Jones &fruits Bartlett hold their Learning, green photosynthetic LLC pigment chlorophyll© Jones in small, & Bartlett bright green Learning, dots LLC NOT FORnamed SALE chloroplasts OR DISTRIBUTION (FIGURE 3.1). Starchy foods likeNOT potatoes FOR have SALE starch OR grains DISTRIBUTION in their cells, and colorful petals and fruits display their pigments as particles or droplets in the cells. Progress was much slower in the study of animal cells. They lack cell walls, and neighboring cells fit together so tightly that even today it is almost impossible to identify individual animal© Jones cells &with Bartlett even the Learning, best modern LLC light microscopes. Fur- © Jones & Bartlett Learning, LLC thermore, most organellesNOT withinFOR SALEboth plant OR and DISTRIBUTION animal cells have no natural color, NOT FOR SALE OR DISTRIBUTION so they remained unseen until microscopists began experimenting with artificial dyes and stains. We now know that all organisms are com- posed© ofJones cells, &and Bartlett that in plants,Learning, each LLCcell © Jones & Bartlett Learning, LLC consists of a cell wall surrounding a small Cell Cell wall Chloroplasts NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION bit of protoplasm. The protoplasm itself contains many diverse organelles, and cells specialize for particular tasks by adjusting the number and metabolic activity of their © Jones &organelles. Bartlett CellsLearning, that spe LLCcialize for photo- © Jones & Bartlett Learning, LLC NOT FORsynthesis SALE OR develop DISTRIBUTION large numbers of chlo- NOT FOR SALE OR DISTRIBUTION roplasts whereas flower petal cells develop various pigments. The same is true of us: our skin cells produce large amounts of the pig- ment melanin whereas our red blood cells fill themselves with© Joneshemoglobin & Bartlett instead; Learning, LLC © Jones & Bartlett Learning, LLC our liver cells have anNOT abundance FOR SALE of organ- OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION elles that break down toxins whereas muscle cells can contract because of the proteins they contain. FIGURE 3.1. This is a view of a liverwort, Pallavicinia, a plant that has a body The© bodyJones of every& Bartlett large organism, Learning, whether LLC only one or two cells thick,© Jones thus allowing & Bartlett us to see details Learning, that are difficult LLC to tree NOTor human, FOR isSALE composed OR DISTRIBUTIONof organs that view in thicker plants. TheNOT white FOR lines areSALE cell walls, OR and DISTRIBUTION each space enclosed are in turn composed of cells. It is possible by walls is a distinct cell. Each cell has numerous green spots, organelles called to imagine a bit of protoplasm or a single cell chloroplasts that contain chlorophyll and carry out photosynthesis (3200) (each growing to the size of a tree or a human but cell is about 40mm wide; a strip of tissue 1 inch long would have about 625 cells it is difficult to imagine it being able to form side by side). © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION. 44 CHAPTER 3 Cells, Tissues, and Organs: The Microscopic Components of Plant Structure © Jones & Bartlett Learning, LLCspecialized parts, each with© Jones a particular & Bartlett structure Learning, and function. LLC Because organisms NOT FOR SALE OR DISTRIBUTIONare composed of cells andNOT because FOR cells SALE can specialize OR DISTRIBUTION as they grow and develop, or- ganisms can become both large and complex. © Jones &Cells Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FORIt is important SALE OR to askDISTRIBUTION ourselves “Where do cells come from?”NOT “How FOR do SALEcells grow OR and DISTRIBUTION develop?” “Do cells die?” All cells originate in just one of two ways: either by (1) cell fusion or by (2) cell division. Cell fusion is the rarer type, occurring when one sperm cell fuses with one egg cell, resulting in one zygote (a fertilized egg). The zygote then © Jones & Bartlett growsLearning, and develops LLC and at some point it© willJones divide & Bartlettinto two newLearning, daughter LLC cells. NOT FOR SALE ORThese DISTRIBUTION in turn grow then divide, and the NOTprocess FOR is repeated SALE manyOR DISTRIBUTION times. Every cell in a plant’s body and in our own bodies can be traced to its parental cell and the one that preceded that and so on back to the zygote. None of our cells is ever produced in any other way. Because we animals have diffuse growth, most of our cells retain this capac- © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC ity to grow and divide, thus allowing us to grow to adult size (our brain cells NOT FOR SALE OR DISTRIBUTIONare exceptional and stopNOT dividing FOR while SALE we OR are DISTRIBUTIONvery young). Plants are different though, and have localized growth. While a plant embryo is still tiny inside an immature seed, cells in the center stop dividing and instead grow and differenti- ate into mature cells of shoot and root; these cells almost never divide again and © Jonesdo not& Bartlett contribute Learning, any further LLC to the growth of the plant.© Jones Cells at & the Bartlett shoot andLearning, LLC NOT FOR SALE OR DISTRIBUTIONroot tip, however, organize themselvesNOT FOR into SALE the root OR api- DISTRIBUTION cal meristem and shoot apical meristem, and it is these Shoot apical meristem Leaf primordium cells that will continue to divide and produce new cells for the plant for the rest of its life (FIGURES 2.2 and 3.2).