256 ORR—TRACHEAL TISSUE IN CAPPARIDACEAE, 
mainly a difference in degree, the varieties of configuration 
seen in the tissues in surface view being brought about by the 
varying depths of the tracheides, which accounts to a certain 
extent for the considerable lack of uniformity among the species 
of Capparis, 
In minor details of structure the differences exhibited by 
the tracheal tissues are more often of the nature of generic 
distinctions. For example, in Maerua (sensu Pax) * the double 
layer of tracheides is distinctive, and has no parallel among the 
other types examined. 
One feature is common to all the species investigated, 
definitely separating the Cappareae from the Cleomeae, and 
that is the localisation of the thickening on the anticlinal walls 
of the tracheides. In the Capparidaceae as a whole, the number 
of species examined is too small to admit of more than a passing 
reference to the speculative value of the sheath characters as 
an aid to classification. So far as can be ascertained, it does 
form a tribal distinction, and among natural allies it has every 
appearance of an additional link of some systematic import. 
Apart from the phylogenetic aspect, the presence of the 
tracheal envelope in the seeds of the Capparidaceae is of much 
greater interest from a physiological point of view. Its structural 
features and position in the seed imply a relationship between 
this tissue and the included embryo which is not historical, 
but definitely functional, and apparently connected with the 
supply of moisture during germination. It is not confined to 
xerophylous species alone, although it reaches a high state of 
development in the seeds of such types. 
It is possible that further researches may lead to a dis- 
covery of the existence of similar tracheal tissues in the seeds of 
other families besides the Thymelaeaceae and Capparidaceae. 
In conclusion, I desire to express my indebtedness to Prof. 
W. G. Craib, for his valuable advice and criticism. 
LIST OF SPECIES EXAMINED. 
Cleome spinosa, Jacq. Cult. Roy. Bot. Gdn., Edin. 
» lutea, Hook. Idaho, cag ae No. 226. 
»  platycarpa, Torr. Californ a, Heller, No. 80f0. 
»  Stockstana, Boiss. Baluchistan (1889), Lace. 
» gigantea, Linn. University Botanic Garden, 
Copenhagen. 
Polanisia eee DC. N.W. Himal., Lace, No. 1188. 
onit, DC. Siam, Khoon Winit, No. 458. 
Isomeris Pia Nutt. Cult. Roy. Bot. Gdn., 
* Pax in Engler u. Prantl, Pfizfam., iii, 2 (1891), p. 234. 
