169 
Pseudotsuga Douglasii. 
Gymnosperms, The nature and function of these cells has been discussed 
at some length by Miss Ferguson (1903-4). In her paper on Pinns this 
writer rejects the idea that these cells are sporogenous on the ground that 
‘ the divisions in this tissue are according to the typic method, and present 
the number of chromosomes characteristic of the sporophyte \ It should be 
remembered, however, that the haploid phase does not appear until the 
division of the mother-cell, and that there may be more than one generation 
of diploid archesporial cells. In his work on St anger ia, Lang (’00) describes 
these cells as a ‘ sporogenous group ’, the outermost of which ‘ form a more 
definite tapetal layer \ Arnoldi (’01) also describes in Cunninghamia an 
‘archesporial tissue ’ surrounding the young embryo sac. 
From what we know of the development and nature of the ‘ spongy 
tissue ’ in the Gymnosperm ovule in the various types studied, there are good 
reasons for believing that it is not only tapetal in function but is also 
archesporial in origin. My belief in this regard has been very much 
strengthened after a study of the development and behaviour of this tissue 
in Pseudotsuga . All of the facts point to the conclusion that it consists of 
archesporial cells which do not reach or go beyond the mother-cell stage. 
Although in Pseudotsuga the tapetum at an early stage consists of 
a single layer of cells closely packed together, it eventually becomes several 
layers thick. It increases with the growth of the young prothallium. This 
increase and growth are, however, not very rapid. In Fig. 10 the prothallium 
is represented in the parietal multinucleate condition surrounded by several 
layers of tapetal cells. It will be observed that the latter are no longer 
closely packed together, but are arranged quite loosely with numerous 
intercellular spaces. It was also observed at this time that the megaspore 
wall appeared as a sharp well-defined membrane of measurable thickness. 
From the time of the free nuclear stage represented in Fig. 9 to the 
multinucleate parietal condition represented in Fig. 10 four weeks have 
elapsed. The development is slow but not interrupted by a resting 
period as is the case with Pinus (Ferguson, ’04). The parietal condition is 
represented in Fig. 11. This is some little time before the primary pro- 
thallial cells are formed. It will be seen that there is a very large central 
vacuole which keeps the cytoplasm closely pressed against the megaspore 
membrane. As illustrated in Fig. 12 the cytoplasm consists of a thin film 
no wider than the diameter of one of the free nuclei which are distributed at 
regular intervals in it. The regular distribution of the nuclei is probably 
due to the fact that they have undergone their last free division and are 
preparing for the mitosis which will result in the formation of walls between 
them. A stage immediately following this division is represented in Fig. 13. 
It will be observed that there is now a cell-wall separating each nucleus from 
its neighbour, and that the cells thus formed are open on the inside and freely 
exposed to the sap of the central vacuole. 
