DICOTYLEDONOUS 23 
Literature. 
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Seaiaiwe:! LO troOducliOn,. tO BOUADY . 5.0 siete oes tec oes 0's eas 52-56 
Member an TOLAUIONS 6 Flic so oo alee w ee slcpie oie ood ses Seb gle « 3-82 
Percen and avis: Principles’ of Botany... 0265.2 c ee 4S 56 
PETES ine ROU INN So ok eae Salant aldveuan dS ul bse Sable end okies hme 365-882 
tray. Wessors and Manual of Botany... ssc... <6 see os et 27-48 
Perse) MIC Ments : OF, BOCAIMY « o.e). Vee) alscaie os 010s, c1e. 6 see. © sim win 2 38 o1 
ipeneem 7. Woundalions: OF ISOTANY. 46 vce cece eo wae ores 2 oe 62-43 
siraspurger: Text Book of Botany........5..+.-.-+---20- 18-27 
Kerner and Oliver: Natural History of Plants, Vol. 1....655-724 
Structure of Dicotyledonous Stems. 
With a sharp pocket knife or scalpel cut cross sec- 
tions of a horse-chestnut twig three years old. Place the 
sections, which should be about 5 mm. thick, on a slide 
in some water and study them under the dissecting micro- 
scope. Observe that the stem is composed of a central 
portion, the pith, which is surrounded by two layers, one 
of wood and one of bark. How thick is each of these 
layers ?- 
The bark is composed of two layers, a brown layer 
on the outside, and a green layer next to it. What is 
the relation in thickness between these layers? 
Lines radiating from the pith may be seen to extend 
to the bark. These lines are called medullary rays. li 
they cannot be readily seen in horse-chestnut, look for 
them in cross sections of a beech twig. About how 
many do you find? Are they all of the same length and 
thickness? They are distinctly seen in quarter-sawed 
oak. Ask the instructor for a specimen. 
Wood tissue is composed of concentric layers, ar- 
ranged around the pith. How many are there in the 
