38 root habits of desert plants. 



Parietaria Debilis. 

 Parietaria occurs almost exclusively in the shade of rocks, or elsewhere 

 where it enjoys protection either against excessive light or severe aridity. 

 The plant closely resembles the eastern clearweed (Spalding: Distribution 

 and Movements of Desert Plants, Carnegie Institution of Washington, 

 Publication 113, 1909, p. 20) and has a very high water content; though 

 growing in the desert it has no characteristics which ally it to typical desert 

 plants. Parietaria varies greatly in size and appearance, a feature depend- 

 ing on water relations more than on any other environmental condition. 

 This may be illustrated by the following observation. In March, 1906, 

 many annuals were seen to be of very unequal size ; the difference was so 

 great in many instances as to make sure recognition of well-known species 

 difficult. Among forms exhibiting this condition, none showed so extreme 

 modification 2iS Parietaria, mature specimens of which, growing under favor- 

 able and unfavorable conditions as regards water relations or protection by 

 neighboring plants or rocks, w^ere found w^hose shoots were from 8 mm. to 



39 cm. long; that is, there w^as a difference of i to 49 in stature (plate 7). 

 The root-system of Parietaria is an extremely meager one. It does not 



penetrate far either horizontally or vertically. The main root of the 

 plant with the longest shoot seen was over 4.3 cm., or about one-ninth the 

 length of the shoot. The length of the main root of the smallest specimens 

 was about 5 times the length of the shoot. The relation of the length of 

 root and of shoot in these instances, together with the general character of 

 the root-system, is shown in plate 7. 



Phacelia Tanacetifolia. 

 Phacelia is one of the most common of the winter annuals on Tumamoc 

 Hill, and one of the most generally distributed, although it may be most 

 abundant and attain greatest development on the northern slope and in 

 the protection of larger plants and of rocks. A prominent tap root is the 

 chief characteristic of its root-system. A mature plant with a shoot 15 

 cm. high had a tap root 2 mm. in diameter at the crown and over 18 cm. 

 long. The laterals of the first order are fine, usually short, and arise singly. 

 Under certain circumstances, however, the laterals become a more pro- 

 nounced feature of the root-system, as, in a younger plant than that above 

 described, laterals 6 cm. in length were seen within 3 cm. of the surface of 

 the ground. With its prominent tap root and fine laterals, Phacelia is to 

 be classed among the plants with an extensive root-system, like the most 

 of the annuals examined (plate 7.) 



Plantago Fastigiata. 



Plantago is abundant on Tumamoc Hill and on the upper bajada at its 

 northern base. It grows everywhere in the open where it is exposed to the 

 sun and is among the most drought-resistant of the winter annuals. The 



