76 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ferentiatod from the primary membrano ; their diameter in transverse 

 section is small. 



The CaryophyllaccaVaro also characterized, with some exceptions, by 

 distinguishing peculiarities in tho structure of the root. The extra- 

 cambial tissue is usually strongly collcnchymatous. The walls of the 

 short cells of the prosonchyma aro thin or collcnchymatous, but never 

 lignified in the entire xylem. 



The root of the Chenopodiaceaj is distinguished by its regular con- 

 centric arrangement. The Cruciferffi include several different types ; and 

 the author appends a clavis by which it can be determined to which of the 

 species examined any given crucifer-root belongs. In other orders the 

 characters of the root are by no means so uniform ; while in other cases 

 those of particular species are very sharply marked off from all others 

 nearly allied to them. This is the case with Urtica dioica and Rheum 

 rhaponticum. 



With regard to the rhizome, the author finds that it generally differs 

 from the root in essential anatomical characters, as in the position and 

 form of the vascular bundles ; and from the stem in the strongly developed 

 cortical parenchyma. A true endoderm in the root was observed in only 

 one instance, that of Helleborus niger. 



(4) Structure of Organs. 



Respiratory Organs. — Herr L. Jost * proposes the term " pneuma- 

 thode " for those parts of plants which are especially adapted by their 

 structure for respiration, such as aerial roots. These are of specially 

 frequent occurrence in many species of palm belonging to the genera 

 Livistona, Phoenix, and others. In L. australis they may rise erect to a 

 considerable height (the result of negative geotropism), and are furnished 

 with an evident root-cap. The " pneumathodes " here are certain white 

 spaces where the ordinary brown epidermis is replaced by cells of peculiar 

 form, containing air, and very loosely connected with one another. In 

 other palms the pneumathodes do not occur on roots rising erect in the 

 air, but on those with a normal horizontal position, or they are found on 

 ordinary lateral roots. It was shown by experiment that the tendency of 

 an abundant supply of water is to promote the production of aerial roots; 

 while, when the supply of water is limited, the pneumathodes are formed 

 beneath the soil. The influence of water on the direction of the growth 

 of roots is, however, indirect rather than direct ; hydrotropism could not 

 cause the roots to rise erect out of the water ; the author considers that it 

 may in great measure be attributed to the properly -named " aerotropism " 

 by Molisch.f 



The structure of the vascular bundle in the pneumathode differs in 

 no respect from that in the other parts of the root ; in the cortical 

 parenchyma the elongated intercellular spaces have almost entirely dis- 

 appeared, as also the epidermis and the hypodermal sclerenchymatous 

 ring, the latter being replaced by a sclerenchymatous layer beneath the 

 peripheral spongy layer of thin-walled cells. 



Further illustrations of pneumathodes are afforded by Pandanus 

 furcatus and pygmseus, Saccharum afficinarum, Cyperus textilis, Luffa 

 amara, Taxodium distichum, and other perennial plants. 



* Bot. Ztg., xlv. (1887) pp. 601-6, 617-28, 633-42 (1 pi.), 

 t Cf. this Journal, 1885, p. 96. 



