102 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



dermatogen gives rise to the epidermis. The procambial 

 strands give rise to the vascular bundles, the inner 

 tissue in each strand being the xyiem, the outer tissue 

 the phloem, while (in Dicotyledons) there remains be- 

 tween xylem and phloem a layer of meristem the fasci- 

 cular cambium, The ground meristem in Dicotyledons 

 becomes differentiated into primary cortex, pericycle, 

 primary medullary rays and pith. The inner limit of 

 the cortex is the endodermis, or starch- sheath layer ; the 

 pericycle extends from this to the outer border of the 

 vascular bundles ; the primary rays lie between the bundles ; 

 the pith is the tissue surrounded by the ring of bundles. 

 The whole of the tissue within the endodermis is called 

 the stele. 



112. Apical Meristem of Stem in Eloclea, Hippuris, 

 etc. The growing point of various aquatic plants is especially 

 easy to study. If possible examine Elodea, Myriophyllum, and 

 Hippuris. 



(a) Pick off the terminal leaves, then cut off the stem tip, place 

 it in water, and carefully dissect away the small inner leaves, which 

 can be readily seen and handled. With the low power note the 

 rather long apical cone, the tip of which is quite smooth, while 

 lower down the young leaves are seen as outgrowths on the sides of 

 the stem, becoming successively larger as we pass farther backwards 

 from the apex. Treat the preparation with eau de Javelle to make 

 it more transparent. 



(6) Cut median longitudinal sections of the apical bud of Hippuris. 

 Treat some with potash or eau de Javelle, and mount in glycerine ; 

 stain others with haematoxylin, and mount in balsam. The pro- 

 cambial tissue is imusually distinct in Hippuris. Farther down, 

 note the development in the cortex of air-passages, interrupted by 

 solid partitions at the nodes and the origin of the young leaves. 

 The older leaves have buds (some may be flower-buds) in their axils ; 

 the leaves bear peltate hairs. 



(c) In sections stained with haematoxylin, or with iodine, note 

 that the meristematic tissue at the apex consists of small cells with 

 thin walls and dense protoplasm these cells have obviously been 

 undergoing rapid growth and division. Farther back, as the cells 

 grow in length, the growth in volume of the protoplasm fails to 

 keep pace with the extension of the cell- wall, and thus the proto- 

 plasm becomes vacuolated, the cell-sap collecting in drops (vacuoles) 

 which later run together to form a large central vacuole, while the 

 protoplasm becomes restricted to a peripheral layer ("primordial 

 utricle") immediately within the cell-wall. 



