PRIMARY PARENCHYMA. CORTEX. PETIOLES. 405 



question, Sect. 7, p. 45, is to be compared ; on the structure of the collenchyma, 

 comp. Sect. 26, p. 119. The thin-walled parenchyma of the external cortex contains 

 chlorophyll, except in the few plants which are altogether destitute of it, and the 

 nearer the surface, the greater on the average is its amount. As already stated, the 

 parenchyma is always traversed by spaces containing air, and often by wide lacunae. 

 Also the large lacunae, chambers, and air-passages in the stems of water-plants 

 (comp. p. 211) lie in the internal parenchymatous mass, and in fact are so distributed 

 that they form a circle, interrupted by radial, usually one-layered lamellae, between a 

 denser zone immediately surrounding the ring of bundles or axial • strand, and the 

 hypodermal zone; or they may be situated in the same region in two or more 

 alternating circles. The former is the case in Myriophyllum, Ceratophyllum, Elatine 

 Alsinastrum, Callitriche, and Utricularia vulgaris ; the latter in Hottonia, Trapa, 

 Hippuris, etc. 



Generally similar phenomena, which cannot be more fully described here, and 

 to some of which we shall have to return in the next chapter, are shown by the 

 cortex of Monocotyledonous stems. Comp. Fig. 120, p. 269; Fig. 125, p. 274; 

 Fig. 171, p. 369. 



In order to avoid repetitions we will return in the following chapter to the cortex 

 of the Fern-like plants, in so far as it belongs to our present subject. 



The stems of those chlorophyll-containing plants in which the foliage-leaves are 

 feebly or not at all developed and in which the stems themselves undertake the function 

 of the green foliage ', and likewise the numerous Monocotyledons with ' halms ' 

 similarly constructed to the foliage-leaves of the same plant, behave differently from 

 the ordinary leafy-stems, inasmuch as their cortical parenchyma assumes the structure 

 of those layers of concentically constructed laminae which contain chlorophyll ; this 

 structure will be described in Sect. 121. Examples of this will be given at the place 

 indicated. Comp. also p. 265. 



Sect. 120. Petioles and stout ribs 0/ Ike leaf show in general a similar structure 

 of the parenchyma surrounding the vascular bundles to that in the stem of the same 

 plants ; the layers and bands of thin-walled or collenchymatous parenchyma, and in 

 many cases of sclerotic elements also, are often continued into them from the stem '. 

 In the smaller petioles and ribs the collenchymatous or sclerotic elements occur for the 

 most part, or exclusively, around the vascular bundles. Aqueous- and chlorophyll- 

 parenchyma take part in the composition of the larger ones in very various forms. 

 The large petioles of Ferns are especially to be mentioned as examples of the band-like 

 or isolated distribution of air-containing parenchyma, covered by an epidermis contain- 

 ing stomata,and lying between masses — here usually sclerotic — covered by an epidermis 

 destitute of stomata. Most frequently the air-containing parenchyma first-mentioned 

 extends down each side of the petiole from the base of the lamina, in the form of a 

 longitudinal band ; in species with a creeping stem, e. g. species of Hypolepis and 

 Pteris, the bands are often continued along its lateral surface throughout its length. 

 in the petioles of Tree-Ferns the bands are frequently interrupted, forming a longi- 

 tudinal row of short strips ; e. g. Cyathea meduUaris. On the persistent base of the 



' [Compare Pick, Beitr. z. Kenntn. d. assimilirenden Gewebes armlaubiger Pflanzen. Diss. 

 Bonn, J?ef. Bot. Centralbl. 1881, Bd. 6, p. 234.] 



' Compare Kraus, Cycadeenfiedern, in Pringsheim's Jahrb. IV. 



