] 25. 



till' pliToin. The .•iir'imm'iMoiil cil tlii' paits ol ilic central cyliiulcr >lii)\vv tlic 

 regular tUHerentiation of tlu' radial humlle ol roots. 



It mijiht at fifst ln' siipjiosetl lluit Poi/onid »/)///(»//( ;.s-.sok/<>', like many plants 

 {Azolla, Hydrocharis, Ji(iiiiiiiciilu.-<, Ficaria) liaviuL; roots with a liniitt'd i;i(i\\tli, 

 throws ott' its root-oap. hut such is not tlu' tast\ sinci' not even the rudinient of one 

 is developed, and the secondary roots break ihroun'h the cortex and ei)idernus 

 without any eovc'rint;- to their tip. Neitiier is the absence of tiie root-cap in any 

 way comparable to the condition found in sonn- Aroids, e. </.. Aiithininiti loin/ijolium 

 ( Bot. Zeitung, 1S7S, p. (>4"). i. where the root-cap is torn away and the root, by the 

 production of a bu<l at its tip. devcdopes into a shoot, and in this way conlinut-s 

 its growth. 



Of the constancy of the [x'culiar condition in I'ni/onid npliiiif//os.'ioi(Ji's. there 

 seems to be no doubt, since it is found to lie true of plants collected in various 

 parts of the United States, and growing under somew hat ditl'erent circumstances. 

 The condition is then something more than accidental. The structure ol the 

 root, so far as the apex is concerned, is then not unlike that of the stem of many 

 water plants { UippuriK mlgdria). where a single layer of dermatogen covers the 

 tip, and inside of this, o-fi regular isodiametric cells of the periblem, which 

 nndergo pericliual division, give rise to the cortex. Inside of this, is a group of 

 4-0 initial cells o4' the same character giving rise to the fibro-vascular system. 



The meaning of this variation and its value to the plant is not certain, liut it 

 is suggested that, since the plant grows in loose sphagnum and the roots are not 

 in any immediate <-ontact with the matei'ial Irom which they draw tiieir food 

 supply, the tip of the root pressing cimstantly against the decaying stems of the 

 moss is a very important factor in the al)sorption of food. Protected, as it usually 

 is in otlier jdants. witli a root-cap, the outer cells of which are not cai)alde of be- 

 coming turgid, the ettic-iency of this part of the root is very seriously interfered 

 with for the absoi-ption of food. With most i)lants, where the whole length of 

 the roots is in imiiiedialc contact with their source of food supply, the W(jrk 

 done l>y the root hairs does not make necessary the use of the tip for absorjitive 

 ])Ui'p(>ses. Neither does Poyonln oph.io</lo!<Hoi(h'!' have the advantages of a water 

 plant, which, likt' Pontederia cra.-isipes and many Moating plants, take up their 

 nourishment from the free water through the agency of the great mass of root 

 h.airs, often so strikingly developed. In PogoiiKi opli io(/lossoide.< only those parts 

 of the ro<it th:it are here and tluM-e in contact with the stems of tlu' si)hagnum are 

 able to take uji food. The necessity' of a large absorptive surface to the root sys- 

 tem is more apparent when we consider that, contrary to the general oi)inion, the 

 bog soil is not rich in nitrouenous material. 



