PURE SPECIES AND HYBRIDS OF JUG LANS. 19 



in the old leaves they may have disappeared from the upper surface while 

 yet some remain on the lower (ventral) side. The awn-shaped trichomes, 

 on the other hand, are apparently confined to the ventral surface of the 

 leaves, whatever may be their age. In this connection an apparent ad- 

 aptation, which is a nice one, is interesting: to note. In old leaves the 

 ventral (or lower) surface is protected in various ways (more especially 

 presumably by the position) and the awn-shaped trichomes are absent; 

 in the youngest leaves, on the other hand, the ventral surface of the leaves 

 is the outer surface and is most exposed and at the time it is well provided 

 with awn-shaped or stellate trichomes. 



In addition to the variation in kinds of trichomes which are present at 

 different stages of development of the leaf, or to the variation in their 

 distribution on the leaf, either of which have only been touched in the 

 foregoing;, another factor also must be taken into account, namely, the 

 variation of the trichomes in size which accompanies their distribution. 

 For example, small secreting trichomes may be larger on the veins than 

 between them, and larger on the dorsal surface than on the ventral surface 

 of the leaf. Also, the trichomes may be larger in one stage of development 

 of the leaf than in a subsequent stage, or there may not be this difference. 

 Whatever may be the causes of the variation in size, the fact that the tri- 

 chomes are variable makes it necessary that trichomes of analogous stages 

 of development, as well as of analogous positions on the leaves, be com- 

 pared, which has been done in all instances. These circumstances, which 

 were noted in the other hybrids, and were observed notably in those of 

 Oenothera, greatly extended the work, but at the same time quite as much 

 enhanced the interest of the investigation. 



JUGLANS CALIFORNICA. 



Four or possibly five kinds of trichomes are to be found on both surfaces 

 of the leaves oijuglans calif ornica (figs. 4 and 5). These are (l) the awn- 

 shaped trichomes, (2) the disk -shaped trichomes, and (3) the long and (4) 

 the short secreting trichomes. Of the third type there may be two kinds. 

 The awn-shaped trichomes measure about 360 / in length; the disk-shaped 

 trichomes are closely appressed to the surface; the long secreting trichomes 

 are 140 p- more or less long, and the short secreting trichomes measure 58 p- 

 more or less in length. A more detailed description of the trichomes, their 

 origin and development, and their variation, will be given in the following 

 paragraphs. 



The awn-shaped trichomes are more variable than the other types, and 

 since there were no apparent causes of the variation this type of trichomes 

 was not especially studied, either in the pure species or in the hybrids. 

 The awn-shaped trichomes are unicellular and frequently are to be found 

 in groups (fig. 4, a). They occur on the ventral surface only and are 

 especially abundant on the young leaves, 



