The cortical tissues gradually shrink and dry up, turning brown and black in patches or all over, and when at length the cambium and medullary ray tissues dry up the whole twig dies off.
They freeze in winter and dry up in summer, and most of them are navigable only during the spring floods; even the Volga becomes so shallow during the hot season that none but boats of light draught can pass over its shoals.
A few shallow salt lakes are filled by rain water, but they dry up on the setting in of the hot weather, leaving a thick crust of salt on their beds, which is used for commercial and domestic purposes.
The year is divided into a dry and wet season, the first from June to December, when rain rarely falls, the streams dry up and the cameos are burned bare, and the second from January to May when the rains are sometimes heavy and the cameos are covered with luxuriant verdure.
There are numerous lagoons in the Llano districts caused by the periodical floods of the rivers, and extensive esteros and cienagas, in part due to the same causes, but these either dry up in the dry season or are greatly reduced in area.