120 MINUTE STRUCT UKE OF THE STEM. 



frequently it is collenchyma. Excellent illustrations of the 

 latter kind of hypoderma are furnished by most Malvaceae and 

 Labiatae. 



363. Schleiden 1 distinguished four types of external cortical 

 layers in dicotyledonous stems: 1. That existing as a perfectly 

 closed layer (penetrated in some cases only by small canals 

 opening into stomata) ; as in most of the Cactacae, Rosa, Begonia, 

 etc. 2. That divided into many bundles, so that the green cor- 

 tical parenchyma reaches the epidermis ; e. g., in Malvaceae, Sola- 

 naceae, etc. 3. That which may be quite distinctly recognized 

 as a special layer, but still grading into parenchyma at the 

 borders; e. </., in Pyrus Malus, Hedera, Fieus, etc. 4. That 

 more completely merging into cortical parenchyma, and therefore 

 less distinct ; e. g., in Populus, Salix, Sambucus, etc. There are 

 some plants in which it is not distinguishable ; e. g., in Cheiran- 

 thus, Mesembrvanthemum, etc. 



*/ 



In Papaver and species of Thalictrnm the cells of the cortex 

 next to the epidermis have thin walls, while the zone next to the 

 central cvlinder may be sclerotic. 



/ *J 



The inner boundary of the cortex of the stem is, as in the 

 root, the endodermis. The thin-walled cells just within it form 

 the peripheral layer of the central cylinder, or shaft. 



3G4. Variations in the cortex consist chiefly in one of the 

 following modifications: 1. Increase of its la} r ers, sometimes 

 to an extraordinary extent, and often accompanied, especially 

 in water-plants, by the formation of large air-bearing intercellular 

 spaces. The student should examine the peculiar structure of 

 the cortex at the nodes, in these cases of spongy cortex. 2. It 

 has been previously shown (215) that collenchyma is a common 

 modification of cortical parenchyma. A variation in structure 

 reaching the same end as collenchyma, namely, strengthening 

 the stem, is found in a great number of plants ; the cortical 

 parenchyma, especially at the outer part, becoming conspicuously 

 sclerotic, and the tissue very compact. 3. Fibres may occur in 

 the cortex, either isolated or in small fascicles. 



3 Go. The primary fibre- vascular bundles of the stem are de- 

 veloped at definite points in the peripheral layer of the central 

 cylinder. Their structural elements, wood and liber, vary as 

 regards their relative amount, even in the same plant. A given 

 bundle may and generally does change much during its course, 

 interlacing here and there with other bundles, and giving off 

 branches at different points. 



1 Principles of Scientific Botany, p. 240. 



