HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 101 



tinuous zone separated from the vascular bundles by several layers 

 of parenchyma e.g. various Cucurbitaceae, Caryophyllaceae, Aris- 

 tolochia, Honeysuckle, and many Grasses, Sedges, and Rushes ; 

 or (2) a continuous zone in direct contact with the vascular bundles 

 e.g. Plantain, many Liliaceae and Iridaceae, Orchids, etc. ; or 

 (3) separate strands, one opposite and in contact with the phloem 

 of each vascular bundle e.g. various Compositae, Labiatae, Legu- 

 minosae, Ranunculaceae, etc. ; or (4) strands scattered without any 

 obvious relation to the bundles e.g. various Solanaceae, Privet, 

 Mallow, etc. Sclerenchyma strands may also be developed in the 

 cortex, between the hypodermis and the pericycle ; or the inter- 

 fascicular ground tissue may become sclerenchymatous where it 

 abuts on the bundles (e.g. Maize and various other Monocotyledons), 

 and these strands may either remain isolated or, more often, are 

 joined up to the pericyclic sclerenchyma. The stereome of the 

 stem may be built up of sclerenchyma bands and strands in any or 

 all of these four positions hypodermal, cortical, pericyclic, inter- 

 fascicular and these may be joined up in various ways. 



11O. Structure of Aquatic Stems. In sections of the stems 

 of aquatic plants, note especially the scanty development of xylem 

 and sclerenchyma, the tendency of the vascular tissue to be massed 

 at the centre of the stem, and the large development of air-chambers. 

 Compare the structure of the submerged stem with that of the 

 aerial flowering stem in plants which send up their flowers above 

 water e.g. Water Crowfoot. Typical aquatic stem structure is 

 also seen in Dicotyledons like Marestail (Hippuris), Water Milfoil, 

 Water Violet (Hottonia), etc. ; and in Monocotyledons like the 

 Pond-weeds (various species'pf Potamogeton), Elodea, etc. 



The petiole of Water Lily (Nymphaea or Nuphar) may be used 

 for the study of aquatic stem structure as regards the characteristic 

 development of large air-spaces. Note the feeble development of 

 the xylem of the bundles, which is practically represented only by 

 a cavity in each bundle ; also the curious branched internal hairs, 

 covered with small crystals of calcium oxalate. 



111. Apical Meristem of Stem. At the growing 

 apex of the stem there is a single tissue, since the cells 

 are essentially all alike. This apical or primordial 

 tissue can be seen in any vegetative bud. On comparing 

 transverse and longitudinal sections taken at successively 

 lower planes, beginning just below the apex, we find that 

 the primordial meristem soon becomes differentiated into 

 three parts, the primary meristems, known as clerma- 

 togen, procambial strands, and ground meristem. 



These three tissues undergo further differentiation. The 



