120 MINTTTE STRUCT DEE OF THE STEM. 



frequently it is coUenchj-ma. Excellent illustrations of the 

 latter kind of hypoclerma are furnished by most Malvaceae and 

 LabiatiB. 



363. Sclileiden'' distinguished four types of external cortical 

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

 closed layer (penetrated in some eases only hj- small canals 

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

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

 tical parench^una reaches tlie epidermis ; e. g., in Malvaceae, Sola- 

 nacese, etc. 3. That which may be quite distinctlj' recognized 

 as a special laj'er, but still grading into parenchyma at the 

 borders ; e. g., in Pyrus Mains, Hedera, Ficus, etc. 4. That 

 more completely merging into cortical parenchyma, and therefore 

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

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

 thus, Mesembrj-anthemum, etc. 



In Papaver and species of Thalictrum the cells of the cortex 

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

 central cylinder maj' be sclerotic. 



The inner boundarj' 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. 



364. Variations in the cortex consist cliiefly in one of the 

 following modifications: 1. Increase of its la3-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 

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

 has liecn previously shown (215) that coUenchyma is a common 

 modification of cortical parenchyma. A variation in structure 

 reaching the same end as eollenchyraa, namely, strengthening 

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

 parenchyma, esi)cciall3- at the outer part, becoming conspicuously 

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

 the cortex, eitlier isolated or in small fascicles. 



365. The i)rimary flbro-vascnlar bundles of the stem are de- 

 veloped at definite points in the peripheral layer of the central 

 cj'linder. Their structural elements, wood and liber, varj' as 

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

 bundle may and generallj- does change much during its course, 

 interlacing here and there with other bundles, and giving off 

 branches at different points. 



1 Priiicii)les of Scientific Botany, p. 240. 



