82 COMPENDIUM OF GENERAL BOTANY. 



even at a very short distance from tlie primary meristem of the 

 aj^ex. The typical dicotyledonous bundle possesses cambium dur- 

 ing its entire life-period. The elements of the vascular bundles 

 of the monocotyledons also had their origin from cambium, but 

 the formative tissue soon becomes changed into permanent tissue- 

 elements, and in regard to the individual bundle this change pro- 

 ceeded centripetally ; the sieve-tube tissue (leptome) and the ves- 

 sel-bearing portion (liadrome) with the accompanying mechanical 

 cells lie in close proximity. In dicotyledons and conifers the cor- 

 responding tissues — namely, the secondary cortex, which is formed 

 outwardly by the cambium, and the wood, formed inwardly — are 

 always separated by the cambium. In winter there is at least one 

 cell-layer of the cambium, which represents the separating boundary 

 between the cortex and wood. The cortex, which is formed from 

 the first or primary meristem of the stem-apex, and which there- 

 fore existed before the appearance of the cambium-ring, is called 

 primary cortex^ in distinction to the cortex formed by the cambium. 

 It does not show the characteristic radial structure seen in the sec- 

 ondary cortex. 



The customary way of speaking of the monocotyledonous bun- 

 dles as ' ' closed ' ' and those of dicotyledons as ' ' open ' ' is rather 

 unfortunate,' for the monocotyledonous bundles are opened toward 

 the fundamental tissue by means of special structural arrangements 

 (transit-cells, large thin- walled cells, etc.), while the dicotyledo- 

 nous bundles form a closed complex by means of the cambium, so 

 that the individual bundle is scarcely recognizable as such. There- 

 fore the fundamental tissue of dicotyledons and conifers is plainly 

 divided into cortex and parenchyma (medulla), a peripheral and a 

 central portion, which have an anatomical-physiological connec- 

 tion through the medullary rays. In monocotyledons the boundary- 

 line between cortex and medulla is also often well marked, at least 

 in the numerous cases in which the bast-ring represents the me- 

 chanical system. From some statements made by Schwendenee 

 on p. 71 of his ' ' Mechanische Princip ' ' it would seem that 

 such demarcation between the cortex and the medulla of mono- 

 cotyledons without a bast-ring is not easily demonstrated. With 

 regard to the radially diagonal course of many leaf -bundles in mon- 



'Nageli designated the monocotyledonous bundles not as "closed," but as 

 " enclosed," which is more nearly correct. 



