34 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. X, NO. 2. 



tions. In these the original structure remains unchanged (PL 2, tig. 18). The hairy epidermis 

 is thin-walled; the cortex (C in tig. 18), consists only of three layers, of which the innermost is 

 differentiated as an endodermis (End. in tig. 18); the pericambium (P) is continuous, and sur- 

 rounds two rays of hadrome alternating with two groups of leptome. 



If we now examine the primary root we notice that the structure of its filiform apex, which 

 only persists during the first season, agrees exactly with that of the capillary lateral roots, 

 which have just been described. But the structure of the swollen base is different. In this the 

 pericambium becomes gradually very active, besides that a cambium develops inside the leptome 

 and extends from there around the primordial vessels. This increase continues from year to 

 year, accompanied by the development of collateral mestome-bundles in the starch-bearing cortex. 

 The mode of growth is like that observed in the main root of C. megarrhiza, though with the 

 important distinction that in this species the apex continues to grow, while it dies off in V. Vir- 

 ginica. Moreover, in the latter species the central cylinder persists for a very considerable 

 period, and the secondary cortex with its coat of cork layers remains active throughout the life 

 of the plant. The lateral roots are quite numerous in both, but in ( '. Virginica they do not 

 traverse the cortex vertically as fully developed roots, but they proceed at once to the periphery 

 until they become free, as is the usual course followed by lateral roots. 



A very long, but relatively slender, primary root is possessed by C. sarmeiiiosa, and it 

 persists for several years. It is not contractile, but combines the functions of a storage and a 

 nutritive root. The increase in thickness is quite moderate, and the central cylinder, with its 

 mass of narrow vessels and groups of leptome, does not break down even in the older roots, while 

 the peripheral tissues- become replaced by layers of cork and starch-bearing secondary cortex. 

 Lateral roots occur, but they are scarce; the primordial tissues are all noticeable in these, even 

 if some, slight increase in thickness does take place; the endodermis is moderately thick-walled, 

 and the pericambium is continuous. 



If we now examine the root system of one of the annual species — for instance, C. a/renicola — 

 we notice that both the slender, main root and its almost capillary ramifications are able to grow 

 in thickness. 



The epidermis and hypodertn are thrown off very soon, together with the cortex, and are 

 replaced by two or three layers of secondary cortex, surrounding the central cylinder of hadrome 

 and leptome, besides some cambium. The lateral roots are all diarchic, but the very irregular 

 position of the numerous (about thirty) narrow vessels in the primary root made the number of 

 primordial rays indeterminable. 



In species where an underground stem becomes developed — for instance in 0. parvifolia, 

 C. asarifolia, and ('. Ohamissonis — the primary root is no longer observable in matured speci- 

 mens, but is replaced by secondary, which proceed from the short internodes. The structure of 

 such roots is very simple, since they show no signs of increasing in thickness, and neither are 

 they contractile. They have an epidermis with many hairs, directly covering a cortex of about 

 five layers, inside of which is a thin-walled, or sometimes slightly thickened, endodermis; a 

 continuous pericambium surrounds two distinct hadromatic rays, alternating with two groups 

 of leptome, 



In bringing these facts together it appears as if the root system of our species of < 'laytonia, 

 represents four physiological types: Nutritive, attachment, contractile, and storage roots. Of 

 these, the first type is well exemplified in the secondary roots of C. parvifolia, C. asarifolia, 

 and V. Ohamissonis ; the second in the annual C. arenicola; the third in the slender lateral roots 

 of ('. Virginica; while a combination of both contractile and storage roots may be observed in 

 the main root of C. Virginica and ('. megarrhiza. In ( '. sarmentdsa, on the other hand, the root 

 seems to combine the function of a nutritive with that of a storage root. 



THE STEM. 



The above-ground stem is of short duration in Claytonia, and lasts only one season. In 

 some species it bears only two large green leaves and an inflorescence (('. Virginica, etc.), in 

 others it bears several, but smaller, leaves, which are either alternate or opposite ( C. Ohamissonis), 



