154 MINUTE STRUCTURE OF THE STEM. 



4. The cortex is developed largely at the expense of the pith. 



5. There is only slight lignification of the elements. 6. There is 

 a great accumulation of reserve materials. 



430. The relations of a branch to the main axis of the stem 

 seldom present any histological difficulties, the tissues of the 

 former being continuous with those of the latter. When a 

 branch breaks off close to the stem, and the portion remaining 

 becomes buried by stem-tissues which are subsequently produced, 

 a knot is formed. 



431. Steins of vascular cryptogams. 1 The following outline 

 indicates the principal points of difference between the stems of 

 Phaenogams and those of Ferns, Equisetaceae, and their allies. 



I. In vascular cryptogams the fibro- vascular bundles are 

 closed and as a rule are concentric. 1 . In Equisetum they are 

 slender and are arranged in a circle. From the median line of 

 each tooth of the " sheath" (see Gray's Manual) a fascicle de- 

 scends perpendicularly through one internode and divides at the 

 one below into two branches, which unite with the lateral ones 

 next to them. 2. In Osmundaceae the arrangement of the con- 

 stituent parts of the central cylinder is not unlike that in certain 

 Coniferae. 3. Lycopodiaceae have the bundles largely dependent 

 upon the arrangement of the leaves, but the axial cylinder is 

 essentially cauline. 4. Ferns proper may have (a) an axial cylin- 

 der, or (b) several concentrically curved bundles. In either case 

 there may also be isolated and rather slender bundles. In both 

 cases above mentioned the bundles coalesce to form a very com- 

 plicated network, which apparently is not dependent for its char- 

 acter upon the distribution of the leaves upon the stem. 



II. In vascular cryptogams the parenchyma in certain places 

 may become largely sclerotic, forming dense and often brown 

 masses, the constituent cells of which are sometimes considerably 

 elongated. 



III. The epidermis in Equisetaceae is strongly silicified. The 

 stomata in these plants are in the grooves ; their development is 

 peculiar in that from one epidermal cell four guardian cells are 

 formed in one plane ; but soon the two outer cells grow more 

 rapidly and crowd down the two inner ones, so that the latter 

 afterwards become distinctly below them. The epidermal cells 

 of Ferns frequently contain chlorophyll granules. 



432. Stems of mosses. Here no true fibro-vascular bundles 

 are met with, but elongated cells fill their place, forming what 



1 De Bary : Vergleichende Anatomie, p. 289 et seq. 



