THE OVULE 275 



There is little correlation between type of vascular system in the in- 

 tegument and type of ovule, except that, in most orthotropous ovules, 

 there is little vascular tissue except a short bundle to the chalaza. 

 Rarely, the vascular supply is strong, as in Juglans. 



In the vascular system beyond the chalaza, no pattern is determin- 

 able. The basal strand may continue unbranched to the tip of the 

 ovule, or may branch pinnately, palmately, or irregularly. The vascular 

 tissue may lie on one side of the ovule only. This distribution of 

 vascular tissue suggests that the strands in the integuments are minor 

 bundle ends, related in number and position to function, and have 

 little morphological significance. 



Suggestions have been made that the vascular system of angiosperm 

 ovules is a remnant of that of the ovules of the pteridosperms and 

 cycads, but no structural similarity is apparent; there is no basic pat- 

 tern in the angiosperm ovule as in the other taxa. A vascular supply is 

 present as frequently in the higher taxa as in the lower; a double sup- 

 ply — considered in the gymnosperms as probably more primitive than 

 a single one — occurs only very rarely. The vascular supply of angio- 

 sperm ovule integuments seems rather to represent, at least in most 

 taxa, an extension of the veinlet system of the carpel lamina, related to 

 the nutrition of the embryo (note its sheathing of the nucellus in some 

 famihes) in ovules in which the nucellus is small and can itself provide 

 little food for the embryo. (Compare the abundance of stored food and 

 the large gymnosperm nucelli with the absence of stored food and the 

 small angiosperm nucelli. ) The opinion that the presence of vascular tis- 

 sue in the integuments is a primitive condition probably rests on its 

 early discovery in the Amentiferae. Absence of vascular tissue in the in- 

 teguments is not necessarily an advanced character; its presence, not 

 necessarily a primitive one. 



Vascular Tissue in the Nucellus. Tracheids have been rarely reported 

 in the nucellus — Casuarina, Asclepias, the Fagaceae, Capparidaceae. 

 The tracheids are annular or spiral, very small and slender, usually 

 isolated or in small clusters. They occur largely in die sporogenous 

 region, sometimes so placed as to suggest remnants of longitudinal 

 series extending upward from the base of the nucellus. In Castanea, 

 they form a loose cluster about the base of the embryo sac; some 

 tracheids extend downward toward the chalaza. In three genera of the 

 Capparidaceae, a few tracheids have been found scattered through the 

 nucellus. The report of tracheids in the nucellus of the Thymelaeaceae 

 is probably in error. Tracheids described as "on the periphery of the 

 nucellus" and "between the seed coat and the embryo" are perhaps in 

 the inner layer of the inner integument, where integumentary tracheids 

 may occur. In Casuarina, isolated tracheids occur occasionally among 



