338 A TEXT-BOOK OF BOTANY. 



acid. (3) When the winged insect is developed nearly all of 

 the cells contain amorphous masses of tannic acid with some 

 adhering crystals of gallic acid. After the insect has emerged 

 from the gall the constituents again undergo change, depending 

 largely on the presence of moisture, when the tannic acid is changed 

 into more or less insoluble products and the galls become more 

 porous. 



THE INNER STRUCTURE OF THE STEM. 



If we make a transverse section of a young herbaceous stem, 

 we observe a differentiation of the tissues, which in several re- 

 spects agrees with that of the root previously described. In the 

 primary structure of the stem the following tissues are to be 

 noticed : The outermost layer is the epidermis with a more or 

 less distinct cuticle ; the second is the cortical parenchyma, fre- 

 quently having strands of collenchyma near the epidermis. The 

 cortex often contains secretory cells or receptacles, and not infre- 

 quently the innermost layer is differentiated as an endodermis. 

 The latter surrounds the so-called pericycle, a sheath consisting 

 of more or less distinct stereomatic strands, either forming a 

 closed sheath or merely representing isolated arches outside the 

 leptome of the stele. Inside the pericycle we observe the mestome 

 strands constituting mostly one circular band (in cross section) 

 in the Dicotyledons and Gymnosperms, or several more or less 

 concentric bands in the Monocotyledons. The mestome strands 

 or fibrovascular bundles may be collateral (Figs. 192-194), bi- 

 collateral or concentric, the last of which being found only in the 

 Monocotyledons (Fig. 195) and Ferns (Fig. 56). 



In the DICOTYLEDONS the collateral fibrovascular bundles occur 

 most frequently and consist of three distinct portions, viz., phloem, 

 xylem, and cambium. The phloem consists of sieve tubes, com- 

 panion cells (or accompanying cells), and cambiform. The last 

 two are thin-walled parenchymatous cells, those of the cambiform 

 being considerably elongated. In addition there may be included 

 in the phloem the stereomatic tissues or bast fibers, which are not 

 infrequently well developed. The xylem includes tracheae or 

 vessels, tracheids, wood parenchyma, and libriform or wood fibers. 



To the student some confusion may arise as to the apparent 

 indiscriminate use of the terms, leptome and hadrome, the former 



