INTERNAL .ANATOMY. The internal anatomy of plants can only be ascertained by the study of thin sections, crossing the body in various planes. and by dissection of the parts. in the lower plants the internal anatomy is as simple as the external form, no differentiation of the cells being observable. In the higher plants, however, groups of cells are differentiated into tissues, and the tissues are arranged into systems. each of which has a particular function to fulfill. These tissue systems are named according to their function. The mnst. important are the following: (1 ) The protective system; (2) the system: (3) the conducting system; (4 I the nutritive system; (5) the aerating sys tem: (6) the secreting system: (7) the storage system: (S) the illechanieal system.
The tissues and tissue systems all arise in an nnspeciniizcd formative tissue. Every plant be gins its development as a single cell. Repeated divisions of this cell and its segments give vise to others essentially similar. For a time these cells retain the same general bum and powers, noteworthy among which is the enpne ity of divis ion. As the cells become older they grow unlike. and change not only in form but in function. Some maintain throughout their entire existence the form and appearance of the youngest cells. These constitute a formative region (meristem), which, by its growth, gives rise constantly to new tis sues and new organs. This primary formative
tissue is found in the larger plants at the ex tremities of the main axis and branches of the roots and shoots, where it constitutes the grow ing points. It is possible, however, for cells which have ceased to divide to regain this power and to resume the character of formative tissue. To distinguish this latter from the primary meri stem of the growing points, it is called secondary meristem, or cambium. Secondary meristem is often formed in one or more concentric zones in the stems of those plants which increase in thick ness as they become older, in the vicinity of wounds, and at various points which cannot al ways be predicted. At these points its activity results in the making of new tissues, or even new organs, in a mailer essentially like that in the growing points. A short distance behind each growing point the cylinder of newly formed tis sues differentiates enough to show three regions: ( t) The outer cell layer. the "dermatogen," which gives rise to the epidermis, and all its structures; (2) the central mass, the "plerome," which develops the central cylinder or stele. in cluding the vascular strands and pith; (3) be tween these, the "periblem," which produces the cortex. The distinction between these regions is permanent, becoming more marked with age.