88 ASSIMILATION AND CONDUCTION 



Bamboo is largely due. In slender underground stems aiding in 

 the attachment of the plant, and therefore chiefly exposed to 

 tension, a much more centralised disposition of the mechanical 

 elements is observed than in the upright shoot. This is well seen 

 in many Sedges and Grasses. An extreme condition is reached 

 in water-plants, where, owing to the prevalent pulling strain, such 

 mechanical tissue as is present becomes concentrated at the 

 centre of the stem (cf. p. 170). 



Most young stems exhibit chloroplasts in the cortical cells 

 (even when these are collenchymatous, as in the Campion), the 

 carbon dioxide requisite for photosynthesis being obtained from 

 the intercellular spaces between them, which communicate with the 

 atmosphere by means of occasional stomata (cf. p. 96) in the 

 epidermis. In some cases [e.g. Umbellifera;) the outer cortical 

 zone consists of alternating bands of mechanical and assimilatory 

 tissues, the former occupying the ridges, the latter situated 

 beneath the furrows on the surface of the stem. Here the stomata 

 are restricted to the strips of epidermis overlying the assimilator)- 

 tissue. The cortex of the stem may even become the chief seat 

 of the assimilatory function ; but this is exceptional, occurring 

 mainly in plants inhabiting dry situations {e.g. Broom). 



Various experiments ^ show that the vessels are the channels 

 by which water and mineral salts are passed from the root into 

 the leaves. The phloem, on the other hand, serves to conduct 

 elaborated food-substances, the proteins appearing to travel 

 mainly by way of the sieve-tubes. If, for instance, the stem 

 of the Vegetable Marrow be dipped into boiling water so as to 

 coagulate the contents of the sieve-tubes, the abundant proteins 

 can be demonstrated by heating longitudinal sections with 

 Millon's reagent. Opinions chffer as to whether the carbo- 

 hydrates are transferred by the phloem, or, as some Botanists 

 believe, mainly by way of the cortical- and phloem-parenchyma. 

 The storage of food-substances in perennial organs (e.g. rhizomes, 

 tubers, etc.) is effected chiefly by the ground tissue, which in these 

 cases is thin-walled. 



Whilst it has been seen that the arrangement of the tissues 

 differs materially in root and stem, the ground tissues in the 

 two organs pass over imperceptibly into one another. The 

 1 Cf. F, & S., pp. no. III. 



