PHOTOSYNTHESIS 



PHRAGMOPEDILUM 



2601 



animals. It may be said, therefore, that life today is 

 dependent upon the green leaf. The first carbon-con- 

 taining compound made is a relatively simple sub- 

 stance, but the first recognizable material is sugar. The 

 crude materials out of which organic substance is made 

 in the cells of the green tissues are CO 2 (carbon dioxide) 

 and water. The leaf green, chlorophyl, and the pro- 

 toplasm of the cell may be regarded as the important 

 mechanism, while the source of energy for the chemical 

 change induced is radiant energy, light. Air ordinarily 

 contains about .03 per cent of CO 2 , yet the ordinary 

 green plant obtains all of its carbon for the making of 

 organic matter from this extremely small quantity in 

 the atmosphere. The chlorophyl is important inasmuch 

 as it absorbs the radiant energy which is directly or 

 indirectly responsible for the process. Chlorophyl is dis- 

 tributed within the cells in definite granules, or small 

 bodies, protoplasmic in nature, commonly ovoidal in 

 form. The light absorbed is largely from the red or 

 red-orange portion of the spectrum. It is possible that 

 the energy so derived is first transformed into electrical 

 energy, yet little is known upon this point. It is cer- 

 tain, however, that green plants are unable to utilize 

 energy derived, for example, from the absorption of 

 heat. The process may be briefly pictured in the fol- 

 lowing manner: The cell-sap absorbs the CO 2 which 

 diffuses into the tissues from the air. By means of 

 the energy absorbed by the chlorophyl bodies, within 

 the cells, the CO 2 is supposed to be reduced to CO 

 (carbon monoxide), and the same means resolves the 

 water into its constituents. The products of these 

 molecular changes form new substances, perhaps for- 

 maldehyde (CH 2 O) and oxygen (O 2 ). The formation 

 of formaldehyde is still somewhat uncertain; but in 

 any case sugar is soon recognized. In all probability 

 the formaldehyde molecules are immediately condensed 

 to sugar (C 6 H 12 O 6 ). It will be noted that the surplus 

 oxygen is in reality a by-product and during active 

 photosynthesis it is produced in such quantity as to be 

 actively eliminated from the plant by diffusion. The 

 usual test for photosynthesis is carried out by counting 

 the bubbles given off from the cut stem of a water 

 plant exposed to sunlight in a well-aerated vessel of 

 spring-water. The content of oxygen in these bubbles 

 is greater than that of normal air, and the rate of 

 bubble-production is a fair estimate of the rate of 

 photo-synt hesis . 



As a rule the sugar formed in the leaf does not 

 accumulate to any large extent, but is transformed into 

 starch. Some of the sugar, however, may be immedi- 

 ately diffused to other cells or 'transported," supplying 

 the needs of this substance in growth. The starch which 

 is deposited is in the form of insoluble granules, and 

 the formation of these bodies on exposure of the green 

 leaf to sunlight is so rapid as to make it possible in some 

 cases to use starch formation as an index to rate of 

 photosynthesis. During the night, when no photo- 

 synthesis occurs, the transformation and removal of 

 the starch usually goes on rapidly, so that within an 

 interval of twelve hours most of that formed during 

 the day seems to have disappeared from the leaf. It is, 

 in fact, changed to sugar prior to transportation but 

 may be removed to other organs of the plant, as, for 

 example, to fleshy roots or tubers, where it may again 

 be converted into starch, accumulating at times to a 

 very considerable extent. 



Photosynthesis is most rapid under those conditions 

 of temperature which are favorable for growth. Under 

 strong light and favorable temperature, however, a 

 slight increase in the amount of CO 2 gives a higher rate 

 of starch-production. The presence in the leaf or stem 

 of other color bodies, such as browns and reds, is no 

 indication that chlorophyl is absent. As a matter of 

 fact, chlorophyl is generally present in such cases, but 

 may be veiled by the more prominent color. In showy 

 flowers, however, chlorophyl seldom occurs. Pho- 



tosynthesis is inhibited by any condition affecting 

 the general health of the plant, and it is low during 

 cold and dark weather. The larger number of plants 

 are most active in the brightest sunlight, but certain 

 shade-loving species are injured by such exposures, and 

 are adjusted to conditions of half-shade, such as obtain 

 in the shade of trees or bushes. g j^j DUGGAK 



PHRAGMITES (Greek, growing in hedges, apparently 

 from its hedge-like growth along ditches). Gramineae. 

 Large grasses, useful for planting in wet places. 



Tall stout perennials with long running rootstocks, 

 strong culms and terminal panicles with the aspect of 

 Arundo: spikelets 3-7-fld. Differs from Arundo chiefly 

 in having glabrous sharp-pointed not bifid lemmas, the 

 long hairs confined to the rachilla-joints, and in that 

 the lowest floret is staminate. Species 3, 1 in Trop. 

 Asia, 1 in S. Amer. and 1, our species, cosmopolitan. 



commftnis, Trin. (P. vtdgaris, BSP). COMMON 

 REED. Culm usually &-10 ft. nigh: Ivs. 1-2 in. wide. 

 Marshes and along edges of ponds. Dept. Agric., Div. 

 Agrost., Bull. No. 20:126. Gn. 31, p. 33. The orna- 

 mental feathery drooping panicles appear in late sum- 

 mer or autumn. A form with variegated lys. is sold 

 under the name P. communis variegata, or spire-reed. 



A. S. HITCHCOCK. 



PHRAGMOPEDILUM (Greek, phragmos, a fence, 

 alluding to the divisions of the ovary). Orchidacex. 

 Terrestrial or epiphytic glasshouse orchids, formerly 

 included (with Paphiopedilum) in Cypripedium. 



Flowers numerous, deciduous, in a raceme or panicle; 

 sepals 3, the lateral united into one organ located 

 beneath the lip, the other back of the column known as 



2926. Phragmopedilum Schlimii. 



the dorsal sepal; petals 3, the lateral similar, the other 

 widely different, forming a pouch or sac known as the lip, 

 the margins all around the orifice reflexed or turned in; 

 column short; stamens 2; ovary cylindric, 3-celled, the 

 walls thick, the ovules borne on placentae on the divi- 

 sions. About 11 species (Pfitzer, in Engler's Pflan- 

 zenreich, hft. 12 [iy. 50], 1903), Trop. Amer. from Panama 

 south. The species here considered were contained in 



