816 SUMMARY OF CURRENT RESEARCHES RELATING TO 



diminution is due to gradual exhaustion of the supply of carbo- 

 hydrates, for if carbo-hydrates are assimilated afresh the activity of 

 respiration takes a new start. 



That this renewed activity is caused by assimilation and not by 

 simple absorption of carbonic acid is shown by the fact that (1) both 

 light and carbonic acid are necessary to the commencement of this 

 condition ; (2) insolation is useless without carbonic acid ; (3) an 

 atmosphere rich in carbonic acid is useless without light ; (4) a small 

 proportion of carbonic acid is sufficient, if combined with light ; 

 (5) absorption of oxygen undergoes increase after insolation ; (6) the 

 intensity of the light is of importance ; sunlight is best ; (7) the more 

 refrangible rays take part in the action. 



Absorption of carbonic acid may take place to a slight extent, 

 in addition to the assimilation ; but it only occurs after a sojourn 

 in a richly carbonized atmosphere, and it passes off in from one to 

 two hours. The solid parts take an active part in the absorption ; 

 seeds apparently absorb as much (in relation to their weight) in a dry 

 as in a soaked state. Dry seeds absorb only insignificant quantities 

 of hydrogen. 



Physiological Functions of the Tissues of Plants.* — G. Haber- 

 landt occupies a section of Schenk's ' Handbook of Botany ' (' Encyklo- 

 pasdie der Naturwissenschaften ') with an exhaustive account of tbe 

 various kinds of vegetable tissue, and of the parts which they fulfil 

 in the economy of the plant ; treating the subject from a Darwinian 

 point of view, i. e. regarding the anatomical structure and arrange- 

 ment of tissues as a series of phenomena of adaptation. The following 

 is his classification of tissues : — 



I. Epidermal System. 



1. Epidermis. 2. Cork. 3. Bark. 



II. Skeletal System. 



1. Bast and libriform. 2. Collenchyma. 3. Scleren- 

 chyma (?). 



III. Nutritive System. 



1. Absorptive System (Epithelium of the root, root- 



hairs, &c). 



2. Assimilative System (Chlorophyll-parenchyma ; pali- 



sade tissue). 



3. Conductive System (Conducting parenchyma ; con- 



ducting bundles [mestome, hadrome, leptome] ; 

 parenchyma-sheaths ; laticiferous vessels). 



4. Aerative System (Tracheal System (?) ; air-conducting 



intercellular spaces with their orifices [stomata and 

 lenticels]). 

 Local structures. Endoderm ; thickened vascular bundle- 

 sheaths ; glandular, oil-, mucilage-, and gum-passages, &c. 



The following definitions are also given : Protoderm consists of 



* Schenk's Handb. der Bot., ii. pp. 557-693 (28 woodcuts). Breslau, Tre- 

 wendt, 1882. See Bot. Centralbl., xi. (1882) p. 158. 



