104 Guayule. 



ducting tissue formed in irrigated wood is undoubtedly much greater than 

 in that of field plants, but the amount of mechanical tissue is also greater. 

 Putting these facts together, it seems reasonable to conclude that the 

 capacity of the conducting elements is correlated with the maximum transpi- 

 ration stream. The relative numbers, and therefore their size, depend 

 primarily upon other conditions productive of the development of me- 

 chanical elements. On comparing the shoots of field and irrigated plants, 

 it is clear that the mechanical conditions in the latter are those under 

 which mechanical tissue would be developed. The mere weight of the 

 foliage alone would be expected to insure such responses. 



ADVANCED SECONDARY CONDITION OF THE HYPOCOTYL. 

 In a more advanced stage of growth nothing of especial note, be- 

 yond that already pointed out, presents itself for discussion. One point, 

 however, is worth noting, namely, that the daughter and granddaughter 

 cells of the cortex remain arranged in tetrads chiefly, thus giving the 

 whole tissue the appearance of consisting of pairs and tetrads of cells. 

 The original, but enlarged, intercellular spaces are very much in evidence 

 (plate 28, fig. 4) . Regularly shaped and disposed spaces, such as have been 

 described for the root, do not occur in the stem. 



AGE AND STRUCTURE IN THE SEEDLING. 



Both popular and scientific discussion frequently turn on the corre- 

 lation of age and structure in the guayule. Inasmuch as the hypocotyl is 

 the oldest portion of the stem, it is worth while to indicate the structure 

 of field plants of known age. A seedling from Station 2, which was less 

 than one year old when collected in April 1909, with a stem 5 cm. long 

 and 4.6 mm. in diameter at the base, shows in the hypocotyl the struc- 

 ture represented in plate 30, fig. i. The living cortex (primary) is very 

 sharply delimited from the cork on account of the rubber-content of the 

 living cells. It will be seen that the specimen closely resembles the slowly 

 grown irrigated plant above described, while in point of fact it is a plant 

 of rapid growth for field conditions, being much above the average size for 

 the locality in which it was collected. It is seen from this, what will in 

 any event be understood, that all field plants are not alike, the water- 

 supply varying at different times in different habitats, thus inducing at 

 times growth quite similar to that which usually occurs under more 

 favorable conditions. This seedling has, in addition to the four primary 

 canals, three series of secondary canals. One below the average size, of 

 the same age, with an epicotyl 8 mm. long, 2.4 mm. in diameter, has only 

 the four primary canals. These are finally thrown out when a diameter 

 of 6 to 7 mm. is attained, and therewith the whole of the primary cortex 

 is lost. 



In a seedling of the same diameter three years old, it is possible to see 

 three annual rings of wood, marked by the larger pores of the first growth 

 of each season. There are in the same stem, aside from the four primary 

 canals, three series of resin-canals, one in the primary and two in the 

 secondary cortex, so that there are marks of three zones of cortex, the 

 primary and two secondary, corresponding apparently with the three 



