190 KANSAS UNIVERSITY SCIENCE BULLETIN. 



cellulose walls which measure 0.001 mm. in thickness. The 

 thick-walled medullary-ray cells constitute approximately one- 

 third of the whole area of a medullary ray. An explanation 

 of there being both thick- and thin-walled medullary-ray cells 

 may possibly be that the thick-walled cells have sacrificed 

 their function of conduction and have assumed that of 

 strengthening. The average vertical length of a medullary- 

 ray cell is 0.01 mm. ; the average width 0.008 mm. In figure 16 

 are shown a few medullary cells as seen in cross section ; in 

 figure 17 as seen in longitudinal section. 



Very little special provision for the passage of food longi- 

 tudinally seems to have been made by Toivnsendia. The ordi- 

 nary collateral vascular bundles are found. A comparatively 

 small portion of the vascular bundle, however, is devoted to 

 phloem — about 12 per cent; in Verbena stricta 25 per cent of 

 the vascular bundle is devoted to phloem. In Mentzelia oligo- 

 speTTiia there is the same proportion of the vascular bundle de- 

 voted to phloem as in Toivnsendia — 12 per cent. Neither sieve 

 tubes nor sieve parenchyma cells are found in Townsendia. 

 The whole work of conduction of food longitudinally is ap- 

 parently assumed by the cambiform cells and the undivided 

 mother cells of the sieve tubes. Presumably the sieve tubes, 

 when present in a plant, would carry the food more rapidly. 

 The most elaborated sieve tubes are always found in plants of 

 considerable length, especially in trailing, clambering and 

 climbing plants. Toivnsendia, however, is so low that it seems 

 that the food can be conducted longitudinally rapidly enough 

 and in sufficient quantities by the cambiform and undivided 

 ipother cells of the sieve tubes without the aid of sieve tubes. 



In figure 18 are shown phloem cells as seen in cross section ; 

 in figure 19, as seen in longitudinal section : a, cambiform cell ; 

 h, undivided mother cell of the sieve tubes. The average 

 length of these cells is 0.18 mm; the average width, 0.01 mm. 

 The average thickness of their cell wall is 0.0006 mm. 



In Toivnsendia, the xylem parenchyma cells and the medul- 

 lary ray, cortex and pith cells afford sufficient means for the 

 storage of food. Upon examination of sections of the stem of 

 the plant collected at the time of flowering and fruiting, there 

 appeared stored up in the tissues of the stem no food what- 

 ever, thus showing that the food must at this time be used 



