When the first cries of "Vamos a empezar!" - let us begin! - echoed over the Pentagon's north parking lot yesterday morning, Jose Rodrigues, a 62-year-old farmworker with a face furrowed deeply by the Texas sun, began his 82d day of walking.

There was a measured patience to his step by now, the black shoes plodding steadily over the concrete, the straw hat low over one eye. Like the marchers who walked with him, he carried a flag. "La bandera," Rodriques called it, and his voice held the passion and hope that a flag can still summon. "La bandera de la union."

The flag was red, colored by hand, with a white circle in the center. Inside the circl stood the black silhouette of a tree - a shade tree, Rodrigues explained, the only soruce of comfort for a worker in the field.

This tree, and the protective embrace of its shade, have become the symbol for a union of migratory farmworkers in a state so difficult to organize that three months ago, desperate for recognition and support, 25 of the union's members set out to walk to Washington.

Their organization is the Texas Farmworkers Union, and yesterday, in a five-mile march from the Pentagon past the White House to the Lincoln Memorial, the workers completed their 1,482-mile journey.

They had collected about three dozen supporters as they made their way through the cities and farm towns of the Deep South, and they were joined for the final leg of their walk by about 400 supporters.

As they had hoped, the marchers arrived at the White House on a holiday that carries a certain bitterness for the struggling little union. Labor Day was officially recognized by Congress in 1894, after the Knights of Labor spent 12 years pressuring for a yearly salute to organized workers.

And "the farmworkers," as TFWU head Antonio Orendain said before yesterday's walk began, "cannot celebrate Labor Day."

Orendain, 47, is a veteran organizer who began his Texas farmworker activities in Cesar Chavez's California-based United Farm Workers.

In 1975, after years of disagreement about the best place for concentrated organizing - Chavez wanted all attention focused on California; Orendain insisted that Texas needed the union just as urgently - Orendain and his supporters broke away to form a new union.

The goals are familiar: guaranteed jobs under union-negotiated contracts, safety and health provisions, grievance procedures and the right - and power - to strike. But unlike California, Texas and the six other Southern states the marchers passed through ar "right-to-work" states.

Under a provision of the Taft-Hartlye Act allowing them to do so these states have outlawed the "union shop," where all employees are required to belong to a union.

The TFWU wants that section of the Taft-Hartley Act repealed, and it wants a national agricultural labor relations board. But more urgently, it wants recognition, both by Washington officials TFWU leaders hope to meet this week and by the thousands of unorganized workers who spend their lives harvesting this country's crops.

And that is why they left the glaring heat of Austin, Tex., June 18, and began walking east. "To make more friends," Orendain said. "Nothing is accomplished by the poor people unless they keep insisting and insisting . . . This is going to be the beginning."

They walked past the rice fields of East Texas and the low swamps of Louisiana. They walked through Meridian, Miss., and through Birmingham, Ala.; through Atlanta and Greensville, S.C., and Charlotte, N.C., and Richmond.

They slept in churches, in school gyms, in a National Guard armory. And as they walked an old green and gray bus rumbled along the roads with them, carrying cots and clothes and the children too young to walk.

They said the farmworkers of the South welcomed them. "They said we were honored guests," said Rita Martinez, a 54-year-old former farmworker from San Juan, Tex., smiling under her straw hat. Families cooked for them - families hardly rich enough to be feeding dozens of strangers - and soft drink and cookie trucks stopped to hand out refreshments.

Gently, sharing an understanding of work and sometimes little else, the marchers traded cultures with their hosts.

In Lake Charles, La., the church kids taught them "Follow the Drinking Gourd," and the song sometimes called the black national anthem, "Lift Every Voice and Sing." The marchers taught them the familiar Mexican song, "De Colores." And together, in both English and Spanish, they sang "We Shall Overcome."

And in Hattiesburg, Miss., a young marcher named Sil Rodriguez remembered, the workers from Texas astonished a crowd of children by going swimming in the public pool. "The kids siad, 'The whites have a pool on the other side of town," Rodriguez said.

The marchers said this pool was just fine, Rodriguez said, and then she taught a new young friend some Spanish. He told her he was a Negro and so people thought he was ignorant, she remembered. Rodriguez taught him the words, and he had learned them by the time they left: "Yo nunca sere ignorante" - I will never be ignorant.