122 COMPENDIUM OF GENERAL BOTANY. 



A third means of protecting areas of intercalary growth may 

 be mentioned, namely, the increase in diameter in the region of 

 the growing zone. Tradescantia erecta, according to Sehwen- 

 dener, is one of those plants in which basal growth of the inter- 

 node takes place ; it has internodes in the form of truncate cones. 

 Exact measurements in regard to the course of intercalary growth 

 have not yet been made. 



With reference to the intercalary (basal) growing leaves, which 

 include the great majority of leaves, it may be stated briefly that 

 the growing areas are protected by the enveloping sheath-like leaf- 

 blades (elongated monocotyledonous leaves) as well as the overlap- 

 ping of the leaves in the bud (dicotyledonous leaves). 



Among conifers there are membranous sheaths consisting of 

 the bud-scales which enclose the leaf -base. 



VI. FOOD-SUBSTANCES DEBITED FEOM THE AT- 



MOSPHEEE. ASSIMILATION OF CABBON 



IN GEEEN OEGANS. 



The dry (solid) substance of the plant-body is, for the most 

 part, the transformation product of atmospheric carbonic acid (CO 2 , 

 carbon dioxide). Pure carbon (C) constitutes about one-half of 

 this dry substance, and is found in chemical union in the cellulose 

 of membranes, in starch-grains, in fats, in plasm, etc. C appears 

 in the green organs as gaseous CO a . CO 2 unites with the elements 

 of water (H 2 O) through the influence of sunlight on chlorophyll, 

 forming starch or some allied compound and setting free oxygen (O). 

 These transformations take place in a very short period of time. 

 The greater part of the plant-body (plasm and cell-walls) is there- 

 fore derived from the atmosphere. 



That carbon dioxide and water form the starting-points for the 

 production of starch as well as for other related substances, with 

 liberation of O, is well known; also that these transformations 

 may take place in a few hours or minutes. But the most discern- 

 ing chemists at present refrain from attempting to explain the 

 individual chemical reactions involved in the important processes 

 of assimilation. In general, the following formula may be con- 

 sidered as correct: 12CO 2 + 10II.O = cC 2 H 20 O 10 (starch) + O 24 , 

 while the gas- volumes remain nearly the same. 



