BOTANICAL SUBJECTS. 305 



descending to very ancient cryptogams, representatives of which 

 are still extant. The essential oil in the Citrus and others, by its 

 diffusion through the air, advertises a suitable plant to a suitable 

 insect, and also may warn off destructive insects. 



One might multiply evidence to almost any extent to strengthen 

 the notion that the oil-glands on the Citrus leaf are no other than 

 remnants of conceptacles or reproductive organs such as we find 

 in seaweeds. 



For instance, Harvey, in describing the foliage of Sargassum 

 vulgare* says, leaves " serrated, strongly ribbed, copiously glan- 

 dular" He also calls these leaf dots " muciferous pores or 

 glands." He does not, it is true, mention that these glands of the 

 Sargassum leaf are in any way connected, in origin, with con- 

 ceptacles, but any one looking at the frond of this Sargassum, and 

 then at that of Himanthalia Lorea,'\ dotted with conceptacles, 

 and also at that of Myriodesma latifolia, Fig. 129 of these Notes, 

 cannot help being convinced that the muciferous spores or glands 

 of Sargassum are the same thing as the conceptacles of Himan- 

 thalia and Myriodesma. The glands of Sargassum are not needed 

 as conceptacles, because other special reproductive organs called 

 receptacles (and crowded with conceptacles) are developed in that 

 plant. 



From atrophy it is an easy step to total obliteration, for in 

 Sargassum hacciferum\ Harvey says : leaves " destitute of 

 muciferous pores, or glands." 



Here, then, are two closely allied plants included in the same 

 genus (Sargassum), the one is dotted with glands, the other 

 destitute of them. What wonder is it that we should find so 

 many plants destitute of glands on their leaves ? They have not 

 only grown out of the stage of reproductive organs on their 

 ordinary leaves, but they have lost cdl trace of them as needless 

 incumbrances. The Citrus, the myrtle, and several others are the 

 ones which continue to retain traces of them, because their 

 remnants have been turned to other useful purposes in the 

 economy of the plant. 



♦ PI. 343, " Phyc. Brit." 

 t PI. 78 

 X PI. 109 



A p. 1724. 



