189 



i 



grows older these flat leaves are gradually succeeded by others 

 that are thicker, less flat, more ascending, and finally by others 

 of a more and more cylindric type until the adult erect cylindric 

 type is produced. Young plants raised from cuttings of the 

 leaves also pass through the same stages, examples of which are 

 illustrated under 19, S.Pearsonii, and 21, S. StuckyL This 

 change would seem to indicate that all the species having the 

 cylindric type of leaf have originated from ancestral forms having 

 flattened leaves. This variability in the form of the leaf, accord- 

 ing to the age, of the plant, renders it impossible to name speci- 

 mens that are not of adult age, therefore in the following key 

 only characters taken from adult flowering plants have been 

 taken into consideration. 



+ 



The key is more diagnostic than is usually the case, because it 

 has been found that one based upon a few briefly stated characters 

 is absolutely useless for working purposes. Also, in the effort to 

 make it as useful as possible, where a species is known to vary 

 in the characters used, it has been inserted in more than one place 

 in the key, in accordance with that variation. Attempts have 

 been made by other authors to discriminate the species by sections 

 of their leaves alone, but this has been found to be quite imprac- 

 ticable, since not only do different leaves on the same plant often 

 vary very considerably, but even the same individual leaf will 

 vary with age or the amount of shrivelling due to drought to 

 which it has been exposed, causing the margins to infold or 

 grooves to appear on the back or sides, which are quite absent 

 when the plant is in vigorous growth. All the leaf sections given 

 in the illustrations are of natural size, made from nature-printed 

 ink impressions of healthy fresh leaves from adult plants, with 

 the exception of those of 5, S. suffruticosay 17, S. rhoaesiana, and 

 29, S. hurmanica^ where sections of old or partly shrivelled leaves 

 are also given to contrast with those of leaves in a vigorous state 

 of health. 



The measurements are In feet, inches and lines; one line equals 

 one-twelfth of an inch. 



N .B.—Detaclied leaves^ living or dead, cannot he determined with this 



Jcey^ ane or more entire growths of the adult plant are required, 



I. — Stem, 1^-5 ft., or wtrhaps sroRK, high (uxkxown in S. hagamoy- 



ensis), erect, branching at or near the base and often above 



THE GROUND, LEAFY THROTJGHOUT OR SCALY AT THE BASE. 



Leaves of adult plants, 7-18 in. long, flattish or 

 concave (perhaps somewhat longitudinally folded 

 in S. bagamoyensis), 1-2 lin. thick at the base; — 



Stem in the dried specimens seen less than ^ in. 



thick; leaves less than ^ in. broad ... ... 1. bagamoyensis. 



Stem §-1 in. thick; leaves ^-IJ ins. broad ... 2. arborescens. 



Leaves of adult plants 1-2| ft. long, 3-1^ in. broad, 

 concave or cliannelled down the face, 6-9 lin. 

 thick near the base : 



Leaves with a channel down the face as broad as 

 the leaf, and tapering to a hard pale brown 

 spine-like point; flower-stem branched 3. Powellii 



