8 



STRUCTURE AND PHYSIOLOGY OP THE PLANT 



Fig. 12. Leaf of apple, showing 

 blade, petiole, and small nar- 

 row stipules. 



S#5^^^. 



The stem structure. 



The internal structure of the plant does not give rise to such definite parts or organs as appear in the 

 external conformation. The plant-body is made up of cells. Some of these cells perform one work and 



some perform another, work. The fundamental tissue 

 is parenchyma. In this tissue the cells are very simi- 

 lar one to another, more or less cubical or equal- 

 sided, or at least not greatly elongated. The vital 

 processes take place in the parenchyma. Out of the 

 ' parenchyma the other and special kinds of tissue develop. 



The special cellular structures in the stem are chiefly mechanical tis- 

 sues of two general kinds of elongated cells, — those that support the plant 

 or contribute to maintain its form and stature, those that transport the 

 fluids. The supporting tissues, giving rigidity to the plant, are of two kinds in 

 respect to the structure of the cell-wall : those in which the cells are thick- 

 ened or strengthened in the angles (coUenchyma, Fig. 24), and those in which 

 the cell-walls are thickened throughout (sclerencyhma). The conducting 

 tissues are also of two kinds : those with trachea-like walls, marked with 

 rings or pits, and those with punctured or sieve-like walls. The supporting 

 tissues may be in the epidermis of young or of small stems, in the bark, or 

 placed inside the 

 woody cylinder. 

 The conducting 

 tissues are usu- 

 ally definitely 

 placed, and these we may consider further. 



The development of these mechanical tis- 

 sues (for transportation and support) results 

 in the formation of vessels, or systems of spe- 

 cialized tissue in particular parts of the stem. 

 Vessel-bearing plants are said to be vascular, 

 in distinction from certain very low orders of 

 plants in which no special tissues of this kind 

 have been developed. 



It is well known that trees of temperate 

 climates and very many other plants have a 

 distinct and separable bark and that they Increase in diameter by " rings " added on the woody cylin- 

 der. On the other hand, palms, grasses, bananas and many other mostly herbaceous plants increase in 

 diameter by means of tissues scattered through the stem ; these plants do not make an annual ring, and 

 they rarely branch extensively. The former kinds of plants were formerly called exogens, or outside- 

 growers, and the latter endogens or inside-growers. These terms are now given up, however, as not 

 expressing good anatomical distinctions. These classes of plants are now named from the cotyledon or 

 seed-leaf characteristics, — the former having two leaves on the embryo plant, and called dicotyledons ; 



the latter having one leaf in the seed 

 or embryo, and called monocotyle- 

 dons. 



In most dicotyledonous plants we 

 all recognize three fairly distinct 

 parts of the stem, at least at some 

 epoch in the life of the plant : the 

 bark, the woody part, the pith. These 

 parts are usually not clearly set off 

 in the minute anatomical structure, 

 however; but we may pause a mo- 

 ment to discuss them. Long tissues, 

 extending lengthwise the stem or 

 leaf, formed of elongated cells placed 



Fig. 13. A compound or branching leaf (Udo, 

 new vegetable from Japan, a species of aralia ) . 

 The loaf at the left is in three parts, each 

 part again divided. 



Fig. 14. Parts of the plum flower, se, 

 sepal; jp, petal (three are shown); sta^ 

 stamens ; os, pistil, in three parts — o the 

 ovary, s the style, st the stigma. The 

 tov of the stem (below o) is the torus. 



Fig. 15. Flowers of a lily, show- 

 ing six leaf parts , six stamens , 

 and one style. 



