THE INHERITANCE OF FLOWER COLOUR IN PISUM 



73 



hilum has an oval shape. Outmost it consists of a hiyer of thick 

 walled palisade like cells (Hpl) sunk in a shallow depression of the 

 external layer of the testa, (see fig. 1 which shows a cross section of a 

 normal hilum), which also consists of thick walled palisade cells (Tpll. 



Fig. 1. Cross section of normal hilum. — Tpl: the outermost 



layer of the testa composed of thickwalled palisade cells. — Hpl: 



layer of palisade cells belonging to hilum. — S." the slit in the 



middle of the hilum. — Tr: tracheal tissue. 



Both the palisade layers decrease in size towards the slit (S) in the middle 

 of the hilum, and finally disappear altogether. An eggshaped or elipti- 

 cal radially disposed tissue of tracheal elements iTr) lies directly under 

 the slit (see also Kajanus 1. c). 



In the seed of the violet flowered plants, however, hilum appears 

 outwardly as a slitlike 

 depression in the testa 

 only, while the above 

 mentioned layer con- 

 sisting of palisade cells 

 is lacking altogether 

 (see fig. 2). The fact 

 that the tracheal tissue 

 is also entirely missing 

 is of still greater con- 

 sequence. The conduc- 

 tion of nourishment to the seeds became difficult because of the absence 

 of the tracheal tissue, and their own development became retarded. The 

 power of germination does not seem to be impaired even if the plantlets 

 are somewhat weaker than those from normal seeds. On account of the 



Fig. 2. 

 viduals. 



Cross section of hilum in violet flowered indi- 

 — Tpl and S same as in fig. 1. Hilum is devoid 



of both palisade cells and tracheal tissue as seen in the fig. 



